Many thanks, Don, for your valuable insight.
Seems that's always part of the problem...finding documentary evidence of what common sense tells you is true.
Guess it's kinda like the observation that there apparently were no pregnant women in the Old West, since photographs of them are impossible (or nearly so) to find.
Not sure how I missed it the first time, but upon re-reading the patent application, it seems perhaps the best evidence so far is "hiding" in the text (starting on Line 68) of Frederick Conway's 1882 patent document, so gratefully located by Prof. Marvel:
I am aware that buckles similar to mine have been made, having loops on either end and a vertical tongue or pin in the center of the plate; but none hitherto have made the loop at one end higher than at the other, in consequence of which the strap will with difficulty go under the loop where it is double, and will be exceedingly loose at the other end....
I am inclined to consider his statement as pretty strong primary evidence that this style of buckle was not only manufactured, but sufficiently common to be known to Mr. Conway, prior to his 1882 application and therefore correct to be used in our period, even though it may be in a somewhat different application.
Ain't history grand.