If you like the layout of the STEN, may I recommend the Sterling?
Same idea, VASTLY superior quality. The magazine alone is better designed than the entire STEN!
Open bolt guns are usually terrible suppressor hosts. The combination of an open bolt, SHORT barrel, and spring weight do not lend themselves to quietness.
Combined with the general lack of ability to find heavier springs (to hold the bolt closed a bit longer/absorb the the increased recoil)...
I've seen both. The 762SD's and M42K did come with one back in the day, but that was several years ago (possibly when they changed from 18T to 51T).
Maybe this is New Old Stock?
It should have a label/sticker on thew outside of the box indicating a mount was included.