The Metropostcard site shows: E. C. Kropp Co. 1907-1956 Milwaukee, WI A publisher and printer that began producing chromolithographic souvenir cards and private mailing cards in 1898 under the name Kropp. These cards were of much higher quality than those that would printed under the E.C. Kropp name. Postcard They became the E.C. Kropp Company in 1907 and produced large numbers of national view-cards and other subjects. Their latter linen cards had a noticeably fine grain. Sold to L.L. Cook in 1956 and they are now part of the GAF Corp. U.S.
Bowers/Martin in "Guide Book of Collectible Postcards" states Kropp was a "highly important publisher of view and other cards for clients across America. The letter N was often a part of the serial number".
I've been going through a box of old postcard club newsletters and I'l lkeep an eye out for Kropp.
