Sorry, but "front base ring" is not a part with which I am familiar.
Sight blades are available. Actually, complete SETS of them are available. You change sight blades until the rifle is putting the bullets at the correct height on the target.
But the "front base ring"...... do you mean the FRONT SIGHT BAND, by any chance? If so, there is a special screw and cross-pin that need to go with it.
Several fellows on here likely have the parts you require, or see Trade-Ex (tan link at 10 o'clock from The CGN Beaver) as likely the quickest commercial source. They have fair prices and fast service.
Check the Sticky at the top of the Milsurps page. You can download a complete set of Armourers' Manuals, parts lists, parts diagrams and operation manuals from the Military Knowledge Library. Or just go on over to milsurps dot com and take out a (free) membership. They have more books for free download than you can read in a couple of years. Serious on that: I downloaded 2GB (equal to nearly 3/4 of a million pages of type) in a couple of evenings. Now I have to read it all!
Hope this helps.
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