Information On This Stamp / Envelope

I'm trying to find some more information on this envelope / stamp. There is a 1903 watermark on the paper.
Based on my extensive (meaning 5-minute) online search, I am guessing #U385. I don't have many envelopes so I don't really know how to deal with it. Do envelopes go together with other stamps or are they generally separately catalogued?
Thanks!

could be a U385 (pink) SC value $3. Lists carmine, pink, or red. seems too light for red or carmine.

US envelopes are found in the Scott US specialized catalog. You can find a cheap, older catalog online!


Peter

Or your local library.

UPSS Die 107, the original "baggage check" die (I don't understand that nickname). Yes, U385 on white printed in carmine (most common), red and pink. Red and pink are priced slightly higher. Listed in the Scott Specialized and in the UPSS 20th century envelope catalog. The UPSS catalog differentiates by watermark, knife, size, color, etc. Scott tends to value by the most common version of the envelope.

Thank you everyone!