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What a find !
CONGRATS ! !
Those Super Grade's are some of the finest rifles to emerge from the New Haven factory !
The correct Winchester Super Grade QD studs (left, below) are 5/16" wide - much larger than "normal" 1/4" wide QD studs (right, below), like Uncle Mike's, etc.
Original SG sling loops came for 1" or 1-1/4" wide slings, are like chicken teeth today, and expensive ($100, +/-) if/when they can be found for sale.
IIRC, sometimes repro/clones show up every once in awhile on E-Bay, but just beware that some sellers pass them off as gennies, at gennie prices.
Regarding the missing aperture, you can:
1) Hunt/shoot through the ghost ring (which I do) - although the aperture's better for target shooting.
or
2) Visit a real gunsmith and ask to coonfinger his stash of apertures until you find one that threads into the aperture holder fully/smoothly - if it looks too "new", it can be distressed with vinegar to match the finish on the remainder of that #98 bolt peepsight.
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