The 'HS' Type 5 slide on eBay that started this thread is a (Riesch) 'Type 6' M2 slide. There are two numbers on the bottom, one of which is 7161843, the M2 slide drawing number. I have no idea what the other number is - maybe a lot number or HS contract number? Also, the pic of the right side is kind of dark but if you look closely you can see the milled ridge at the top edge of the box.