I purchased this lens for $5 at a thrift store. It's fully functional and the glass seems in good condition, however, I cannot identify the mount or the lens (the front name ring is missing).
When I searched F=1.9 and 50mm, all I found online was a Chinon lens with a Pentax K mount. This is not a K mount, as I cannot mount it on my Pentax bodies and adapters. I couldn't mount it on Canon A-1 or on Minolta MD mount either.