Should be no problem finding one. I just checked my own Winchesters. I too, own a post 64 (1984) XTR featherweight (257Rob) and Checking my M-70 gen-3 Supergrade I find they are Identical. (except the S.G. is metal and the other plastic.)
Part# U354520190 Fits SuperGrade grip cap, metal (blued) $11.75
Part# U354520150 Fits Sporter, Featherweight, super Express grip cap, Plastic $1.50
This is from '08 parts catalog, call Winchester's (Browning) parts Dept and they can be ordered.
Hope this helps. CRASH87