Quote:
One of the more fascinating and challenging postal history ideas, is to build a collection of the four imperf stamps on cover as singles and pairs to pay various rates to various destinations
I will take the bait for a tangential post with a few covers I had handy.
There are numerous FDC's of the sheets, and to some extent the singles, but on-cover uses beyond the FDC are far more elusive - even philatelic ones
August 4, 1936. Bottom pair paying the 6 cent single-weight airmail postage on a double-weight mailing and getting caught for the extra weight/postage!

June 4, 1936. Pair of Tipex sheets on a registered cover from the Philatelic Agency. This cover could be correctly rated as either double rate (0.06) plus 0.18 registry fee for $5.01 to 25.00 indemnity, or triple rate (0.09) plus 0.15 for up to $5.00 indemnity. Despite having a philatelic "taint", These Agency covers often show some incredible uses.

June 12, 1936. Tipex sheet plus 8 cents on cover from the Philatelic Mart of Chicago to Germany, total of 20 cents. Rated at 0.05 surface postage plus 0.15 basic registry fee.

Undated, c1936. Single stamp on a double-rate printed matter mailing to England. Rated at 1.5 cents per 2 ounces, then doubled for item of 2-4 ounces. Flap sealed, envelope slit at both ends to create a wrapper.
