Stamps On Covers From 1911-1915

I have about 60 5 cent blue Washington stamps mailed from Camden N.J. still on original envelops and I am trying to determine if they are 335/378 without removing them from the covers to determine what the watermark would be. Is or are there any other ways to ID them or am I going to have to remove them from the covers. I would prefer not to remove them but I guess I may not have a choice. Thanks in advance for any help.

The Morley-Bright Roll-a-Tector sometimes worked for this, but they are no longer manufactured, and if you find an old one, remember that the sachets had a limited shelf-life. I liked using them, but have encountered strong differences of opinion. There is some good discussion in the thread about watermark detection.

335 was issued Dec 1908 and 378 was issued in Jan 1911. As your correspondence begins in 1911, most likely the covers bear 378, unless the sender or the Camden N.J. post office had a supply of remaining 335s.

And to the community!

Many thanks to both Seigaku and to littleriverphil. I will be checking out the thread on watermarks. Thanks again.