This clamp will fit both flat and round light mounts as on the "Never Out" brand. Stem covers measure approx. 7/16 ID on the 2 small ones and 9/16 ID on the large one. $16.00 with FREE shipping in lower 48. Phone 720-289-2896
The left most dust cover looks like it would fit a Schrader 888 stem (like a 777, only 1/4 inch shorter).
The center dust cover looks like it would fit the current (too large) reproduction metal stems.
The right most dust cover looks like it would fit a Schrader 777 stem (original equipment on Model Ts).