The Walther MP is a completely modular design: by swapping out the upper, one can create an MPK or MPL. There are very, very--very--few fully-transferable Walther MPs in the USA; you can probably count them on one hand. The others are almost all LE guns or dealer samples imported after 1968 and therefore restricted to NFA licensees.
It is a very competent and well-designed SMG with an excellent magazine, but not many were sold, leaving Walther with an enormous inventory.
In the 1980s --much too late-- Walther attempted to develop a conversion that fired from a closed bolt in order to compete with the H&K MP5. Unlike the H&K which was roller-retarded and hammer-fired, the Walther conversion was a striker-fired blowback that--because it utilized advanced primer ignition-- lacked flexibility with different ammunition. It could be tuned to work reliably either in semi-auto or full-auto, but not both, at least not easily. Plus the receivers were already machine guns, so the conversions could not be legally imported for commercial sale anyway. The MP project died a lingering death.
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