Dating 40 Slagelse
Just the three sound 15 re stamps would sell as used stamps alone for 6 (9/08)! C1-4, #210 [10, 15, 25, 50 re 1925 Airmails; 1930 5 re Christian X 60th Birthday] (VF, F-VF) on 28 June 1931 registered, printed matter, airmail CATAPULT cover from "KBENHAVN LUFTPOST 1" to the U. (Long Island City, NY; within the NYC delivery zone thus no additional airmail postage required). C7 x2 [15 re 1934 Airmail] plus #221 Type I and #226 (all VF) on 7 re (Fabr 128) 28 September 1939 postal card from BORUP, marked "Via Sverig" (Sweden) to the U. C7, B12, 278 (VF) on 28/29 January 1942 ICE MAIL cover carried by D. Though there was a time when such covers were not uncommon, I now only rarely see them.
Blue Danish LUFTPOST label from the upper right corner of the pane, which thus bears the print date inscription. C7 (VF, but gum soak stain) solo usage on 6 September 1938 cover from TLLSE to MNTT, FINLAND with receiver. Also with red pictorial "flying trunk" handstamp (nice H.
IMPERFORATE upper left corner margin example on 17 July 1930 COVER along with two Denmark #C2 [1926 15 re Airmail] with KBENHAVN machine cancellation and airmail LUFTPOST/2 hand cancellation. To Malm, Sweden, with LUFTPOST airmail 17 July receiver postmark on reverse.
Larger (9 x 6.25 inch; 230 x 160 mm) envelope, however still fits nicely on an album page.
While not a rarity, this is scarce; I believe that this is the only imperforate example I have had on cover in 41 years.
Lning states that while 4500 perforated stamps were issued, only 500 imperforate were issued -- very scarce in any condition; extremely scarce on cover.