We all know that cachets add value to first day covers. However there was one cover servicer of the thirties through the sixties who went against the trend and issued his FDCs uncacheted. Today they are the only uncacheted covers of the era that command a bit of a premium over other uncacheted FDCs of that time. Why this is so can only be explained by the oddity factor (most uncacheted FDCs are produced by collectors) combined with Adam K. Bert's renown in philatelic circles.
As in stamp collecting, there are many variables at work than in collecting covers -- perhaps more than with stamps. As the stamp collector must be knowledgeable in what gives his stamp a premium value, so must the cover collector be aware of the story behind a cover, as well as many other factors that make it a desirable cover.
Adam K. Bert turned his back on the cachet trend as it was heating up, producing plain FDCs featuring only the stamp and the FDOI cancel. Even the size of his covers was going against the tide. The classic #6 cover was ignored for a smaller envelope, not conducive to a normal sized cachet.
Today the biggest seller of uncacheted FDCs is the USPS. Those who produce add-on cachets take advantage of these already serviced covers. There is always a great stock of the classic old Bert FDCs on the market that might be good for add-ons were it not for their unusual size. Bert had a good number of customers, so these classic Berts are far from scarce -- it is possible to find a lot of a hundred for around $50. Perhaps as a FDC servicer he had a vision. But time has proven that it was not one that current FDC collectors appreciate.