Poland German Occupation Hitler Definitives Sc# N76-90 Imperforate

the set I purchased was listed as "F/VF, no gum"
My question is does anyone know if these were possibly issued this way or are these stamps more likely to be forgeries or tampered with ?
How does "no gum" happen ?
The set was VERY reasonable in price compared with gummed version and I'm having second thoughts.

Unused no gum means that the stamp was either used for postage, but not franked, or, more likely for this period, had been placed in an album using the gum, and the stamp soaked off the paper. Some collectors also used to - may still - remove gum to prevent the deterioration of the stamp, especially in certain climatic conditions. Nothing suspicious, in most cases.

Thanks very much Geoff. I forgot to mention in original post that this set was the imperforate variety.

The imperf General Gouvernement Hitler heads were NOT officially issued but were unfinished stock that the occupying Soviet troops looted from the printers in Vienna in 1945 & then put on the philatelic market. The current Polish Fischer catalogue has the 12 common ones in the set listed at about $5-$6 each, the other 6 will set you back $500+ each.

The copies I have are gummed but I don't have any info if any of the unfinished stock was umgummed.

There are a fair number of faulty imperfs around (partial gum, face thins, gum soak stains, etc.) that suggest some were badly stored or handled at one time. Someone then soaked stuck-together sheets apart probably yielding the no gum ones. If the backs are very, very smooth it would suggest that they were from sheets that were never gummed, though.

Thank you both these replies are very helpful.
ebay can be a tough place to do business, you have to judge from photos, etc.
But in New York City we really only have one walk-in stamp shop left. (hard to believe, eh ?)
Thus ebay is the most convenient and most times best-priced place to obtain the material I'm interested in.
Again, thank you both Yea Polska & hy-brasil. I'm better informed and feel much more comfortable with my purchase.