Thanks to all for the replies. It's good to know it's not my imagination.
For information, I drafted an e-mail to Marble's stating the problem I was having with the tang peep windage adjustment. I received a response within three hours providing mailing instructions for repair. :-)
Prince of Wales: Marble's does provide a tang peep for the Winchester Model 64, but the parts/order numbers are different for each model (even Browning/Mirouku reproductions of the various Winchesters). Make certain to check at the specifications page on the Marble's website before ordering.
In visiting Marble's website preparatory to drafting the e-mail, an additional question was been raised. In reviewing the specifications page for the peep tang sights, I noted that there are two designations (and differing part numbers) for the Browning Model 71, specifically:
"Browning 71" (number 009828); and
"Browning 71 LT" (number 009822) - with a notation, "Made in Japan".
There is confusion regarding the correct Marble's peep tang sight for the Browning 71 rifles. All the Browning Model 71 rifles were "Made in Japan" and all were stamped with Model 71; there has never been a catalogued Browning Model "71 LT". (There were four variations: 20" carbines and 24" rifles, each offered in a standard or a deluxe grade. Frame dimensions are the same for all these; all featured pistol grip stocks; none featured any tang safety; none was designated "LT".)
Before ordering, I contacted Marble's tech support (at Brownell's) and was directed to order the Marble's # 9828, which I did. No one at Marble's, Brownell's, or Midway (each of which list models for the mysterious "LT") could explain why there was more than one tang site variation for the Browning 71. On the Midway site, in the customers' ratings section pertaining to the Marble's tang sights, one customer reported that his gunsmith, when mounting the sight (it requires one hole be drilled and tapped in the tang) noted that the elevation stem provided was not correct for the Browning 71 (though, presumably, the order/parts number was correct). Thus, there may really be a problem with the dual parts numbers.
A continuing mystery!