Martin, welcome to AAPK.
Do you have a #53 two blade Jack. I have two with no pattern number stamped on the tang. The only reason I know they are a #53 pattern is I saw a picture, here while back, of one that was stamped 53 on the pile side of the tang. Those are the only three I've ever seen. I can't find anything on these knives. They aren't in any of the catalogs (online) I have access to. They have a long line stamp so they had to be made sometime between the middle-late '50s to some time in the late '70s - early '80s. Since the handles are that black synthetic material, I feel reasonably more like the middle '50s - early '60s.
I understand this is not what you were looking for but I'm grasping at straws here and you look like a good source to mine.
Forged on the anvil of discipline.
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"The #72 pattern has got to be pretty close to the perfect knife."
--T.J. Murphy 2012