Identification: Scott's #335 Or #378 On Cover

Attached is a blue 5¢ Washington, scan on cover (actually a pc). Perf. 12, left straight edge. - I looked over a number of Scott's #335 and #378 on cover for sale on the internet. Perplexed: how do these sellers know which of these two stamps they are selling without knowing the watermark? All dated after 02/11. Any guidance/information is appreciated. Am I missing something? I am aware that stamps can be removed and then replaced. Could this be a common practice for stamp identification?
Respectfully submitted:
jrc531 - the 40 watt bulb in the gymnasium

The only way to know for certain would be to lift the stamp and check for the watermark. A March 1911 cancel would tend to make it a 378, but it still could be a 335.

Mr. Gator, as usual, I sincerely appreciate your time and knowledge. Thank you...
Respectfully submitted:
jrc531