"Looking for information on my rear sight mount. The measurements are 13mm x 10mm and the hole is 3mm x 8mm. Thanks to Lancaster for posting this for me."

must be british made, please make also pics of the front mount because they are calibrate to each other. just now I can only assume the scope mount feet that fitss into this hook into and than locked into the front mount. so there must be an undercut in this part here.