Hi Louis
It could be from a Titan or Falcon I suppose, Now you mention it, It does actually look like a the stock on a 19 that I used to have!
I have a Mercury that was modded by Ed Bishop, It's the nicest standard Merc (As in non Challenger or 'S') that i've owned!
It's been changed from a crosspin breech to a bolt and nut for a more definite lock up!
Cheers, John