Hand-Written "Rec'd" & "Ans'd" Dates On Covers Question

Those perfs seem very clean for a soft paper stamp.

Possibly. Revcollector, is it possible (or advisable) to soak only the area of the cover where the stamp is applied so that the stamp can be taken off and examined without damaging the cover?
It's difficult to use a thickness gauge as is, because there seems to be light adhesive over the stamp and I want to avoid any false readings.

I have another stamp that I'd like to examine as well. If it has a single line watermark it would make it a #375 and beat the current EKU by a day. Unfortunately, it's on a very thick postal card:

In my opinion removal of a stamp from a cover should only be done by an experienced professional, and then only if the reward is worth the risk.

My 2024 Scott Catalog shows the EKU for 375 as December 3, 1910.

Regardless, one way you can try to identify the watermark of a stamp on card or cover is with a Morley Bright watermark detector. It includes a small sealed envelope of a blue gel that you press against the stamp and then squish the gel on top of the stamp using a credit card or stiff plastic. The gel will collect against the thinner areas of the paper, revealing the watermark. The gel stays inside the sealed plastic and never actually touches the stamp, so there is no risk of damage.

I used one about 20 years ago with mixed results. If you can find a cheap one on ebay it is worth having around so you don't have to soak an EKU stamp off cover making it ever so much harder to get certified.

Zebraman - That was quick! I had this saved a couple years ago, and someone already beat it by a lot! Thank you for the update.
I will look online for a Morley bright. It's one of those things that appears online ever so often.

M and m- I agree. But I also enjoy learning about and practicing conservation/verification techniques myself. That's the most fun aspect of the hobby in my opinion!