Can anyone tell me what kind of rear sight would fit the base on this Remington 40X listed on Gunbroker? I can't give a link but the listing number is 619065369.

I did not go look at the listing, but I doubt it would be different that any other 40X- Red field base for International or Palma would be the normal choice.

If it isn't that, then perhaps a more modern base for a modern micrometer sight like a Warner etc. Lipski makes such a base and is adaptable for most older ( RF) and newer (Warner, RPA etc) sights.

HP Master hit it on the head. That rifle is set up for LR prone matches. It would use a Warner type rear sight and it has the barrel block for the Unertl type scopes.

Same as a 700 also.

As mentioned the Lipski base which attaches to a rear scope base provides almost unlimited adjustment for the height or position of the micrometer rear sight. Basic Weaver aluminum rear scope base for the Rem. 700 works fine.