The MPL and MPK were developed in the early 1960s, and were based on the earlier Italian Franchi LF-57. The MPL was offered as a long-barreled model and the MPK was the short-barreled model. Both models entered production in 1963. They were adopted by the West German Navy and Federal Police as the MP3 (MPL) and MP4 (MPK), and also saw sales to various South American and African countries. Production ended in the mid-1980s.
The MPL and MPK were blowback-operated submachine guns. They utilized an "¬"-shaped bolt with the long section carried over the barrel. The firing pin, extractor, and feed horn were fixed to the short arm of the bolt. The return spring was carried on a guide rod that ran over the bolt and across the length of the body. The cocking slot was located over the barrel, and the cocking handle was non-reciprocating, only engaging the bolt when it was manually retracted.
The construction of the MPL and MPK submachine guns were cheap, as both were largely made from stamped steel. Both had safety switches but no fire selectors, and could fire full-auto only. The flip-up sights were adjustable for 100 to 200 meters on the MPL or 50 to 100 meters on the MPK. Both models would be fitted with a detachable suppressor.