Construction of the Project 620 corvette Kaszub had been plagues by delays, quality problems and unavailable materials. Even once completed to a broadly combat-worthy spec in 1991, it did not bring much to the Polish navy except from a green-water training platform.
The quality of the design was nevertheless recognized, and the Polish Navy pushed for further construction of an enhanced design. The design was also pitched to the Warsaw Pact Military-Technical Committee as a candidate for a new coastal defense corvette. Negotiations followed, and in late 1992 a joint design committee was set up between Poland and the Soviet Union. The multi-role corvette originally planned could not be developed due to volume and technology constraints, and two different variants were defined under Project 621.
The first derivative versions retained the hull and most of the superstructure of the Kaszub. The large area behind the mainmast, originally set aside for an Osa SAM system, saw the installation of two Uran AShM quad launchers, giving the corvette a significant anti-ship punch. In addition, the anti-air weapons suite was brought up to late 90s standard with the installation of two gun-only Palma CIWS and a single Sosna-F short-range SAM launcher aft. The sensor suite was upgraded accordingly, with a Pozitiv-series air/surface target acquisition radar and a Garpun/Bal surface search/acquisition complex for the Uran missiles. The complete ASW system was retained, making this version effectively multirole, though lacking in detection range. Despite the additions, the new version remained sea-worthy enough for the Baltic Sea, and the first ship was accepted into service by the Polish Navy in 1994. Three ships were delivered to Poland, where they served alongside the capital units and often operated together with missile boats in composite squadrons. Five ships were ordered by the Soviet Navy to complement their Baltic Fleet coastal forces, though external observers suspected that the contract was mostly meant as financial support to the Polish shipbuilding industry. Whatever the case, the Soviet boats served a long if uneventful career in the Baltic Fleet, being deployed in several coastal defense units out of Kaliningrad and rotating out to Polish and East German bases. The required weapon and sensor systems were cleared for license production in Poland, quickly helping in the build-up of the Polish military shipbuilding capability.
Still based on the Kaszub structure, Pr.621R did away with all equipment on the aft deck to make space for a helicopter landing pad. The helicopter of choice for the Polish Navy was the W-3 Sokol, the size of which prevented the installation of a hangar on the Pr.620 hull. The Kamov Ka-28 was rejected on grounds of operating costs, and a dedicated ASW variant called W-3PL developed with Soviet assistance. Equipped with an on-board helicopter and the original ASW suite, this variant of the class had an exclusively ASW mission set, lacking surface attack capabilities and with a more limited air defense than its multirole cousin. Entry in service of the class was delayed until the new helicopter was tested and delivered to units. The Polish Naval Air
Despite its limited offensive capabilites and detection range, this sub-class was considered quite successful, and saw regular service in the Baltics under both Polish and East German colors. Several allied and unaligned countries with brown-to-green-water navies also invested in this variant to ease their transition to the more complex combined operations required by modern ASW warfare. The limited displacement, rugged platform and easy-access handling system made it an ideal candidate for budding navies, while the helicopter platform and reduced weaponry granted it more flexibility, including in peacetime public service missions, than most of its Warsaw Pact counterparts. The Soviet Navy declined a purchase of this version, considering its helicopter capabilities insufficient in comparison to larger frigates, and not advantageous in the very dense sea- and air-borne structure of Soviet coastal defense. They would nonetheless have a hand in the development of follow-on variants.
Still unable to decide on a new-generation small anti-submarine ship (MPK) design to replace the uncountable but obsolete 1000-ton ships of the Pr.35, .50 and .159 classes, the Soviet navy turned to the Polish shipyards for proposals in the late 1990s. The Pr.620 hull was well-considered for its ruggedness and stability, and the projected systems then in development were regarded as enabling a full frigate-like ASW suite on a 1500-ton platform. Key to this, besides steadily miniaturizing LOS and satellite data-exchange links, were the further developments of the RPK-9 Medvedka ASW missile system, as well as the dedicated ASW variant of the lightweight Ka-226 helicopter.
Once again, a joint design team was established by engineers from Gdansk and Zelenodolsk, which came up with a significantly upgraded variant of the legacy Pr.620, designated Pr.622 once in service. While the general structure and most of the foredeck were retained, the central superstructure was brought together around a small hangar for the Ka-226P, located as close as possible to the center of gravity of the ship. Sea-keeping was enhanced by lengthening the hull slightly, though active stabilizers were never fitted.
With the removal of the torpedo tubes, missiles and stern AA weapons, most of the stern deck was dedicated to a full-size landing pad. Tests would allow landing and re-supply of the 10-ton Ka-27, although it would never fit the hangar. The offensive weaponry was trimmed down to a pair of retractable quad launchers for the RPK-9 Medvedka ASW rocket, which would later evolve based on the URK-11K multi-role weapons suite. Self-protection was afforded by two Palma CIWS and one of the first operational fits of the A-222M twin 57mm autocannon. The sensor suite was austere but commendable for a ship of that size, featuring both hull and towed medium-frequency sonars and a lightweight 3D surveillance radar. Later ships would received improved systems along the way, and before 2010, production switched to a slightly revised Pr.622.2 version integrating brand-new systems developed for the Project 2133 and 2168 frigates. By 2020, 25 ships of both variants would have entered service in the Soviet Navy. The Polish forces never showed interest in the design, having renewed their large surface combatants in the 2000s.