It appears to be an Israeli Defense Force (IDF) K98 sniper scope mount 'adapter' for the Nimrod 6x40mm scopes in the late 1970s? or perhaps early 1980s.
My historical understanding: In the 1950s the IDF used FN-made K98's with a little green 4x Wilde-Herbrogg scope in a unique Israeli-made claw type mount.
At a later date (probably late 1970s or early 1980s) the IDF updated these optics to the 6x40mm 'Nimrod' scope, but they needed to fabricate an adapter
of some sort given the claw mounts were permanently mounted on their K98 sniper rifle receivers. These adopter scope mounts have 26mm rings to use w/ the Nimrod scope. Apparently, these adopters were sometimes welded to the old claw mount to enhance durability, as I presume they would come loose otherwise (See welded areas in 1st pic). The rings are obviously welded to the mount as well.
Unfortunately IDF K98 sniper rifles with that unique 'claw-type' scope mount are quite rare in the USA, but attached are pics of an 'early' IDF K98 sniper (brown stock w/ green 4x Wilde-Herbrogg scope) and a 'later' K98 sniper rifle (painted stock w/ Nimrod scope). I've not seen these rifles in the USA, and I'm not sure if that collector is in the US, or located somewhere else, but maybe some were imported back in the day. You can see your mount being used on the top rifle (with the tan painted stock). I suspect the 3 digits on your mount were the last 3 digits of the serial # of the K98 it was once fitted it. Pretty sure the rings are European spec 26mm diameter (not 1" inch). I suspect these mounts were likely made by the old IMI entity.
https://en.wikipedia.org/wiki/IMI_Systems
That's my best guess re your somewhat obscure scope mount. I hope that helps.
