Hi, Tax Phd,
If you mean a receiver that was registered before 1986, you are correct. But the 1986 law did not incorporate an amnesty period, so any machinegun/receiver not registered before that is illegal. Most STENS on the market are either a few tubes or tube guns made and registered prior to 1986, or amnesty registered guns. Maybe CATCO or someone has semi-auto parts, but AFAIK, all the parts kits are from standard selective fire STENs, so using them to build a semi would be both difficult and of very questionable legality. I doubt BATF approval could be obtained.
Also, it is of interest that the serial number on a STEN is not on the receiver, it is on the mag housing, which normally cannot be detached from the receiver tube, but is still not the receiver.