Is This 260 Or 275? Couldn't See The Watermark

The previous owner marked this Stamp as 275, but I couldn't find the watermark in the fluid. What's your thought? Besides watermark, any other difference between 260 and 275? I read somewhere that the color is slightly different. Anyone can help?

As far as I can tell the watermark is the only way to tell. I used all of the identification sites I'm aware of - philately.pbworks.com - I have never used watermark fluid. Do you need to shine a bright light in order see it?

Orange stamps are notorious hard to see watermarks. You probably will only see a watermark during the very short (second or two) as the fluid flashes.
Don

I know it will sound/look strange - but I believe the watermark on your stamp is only visible on the top far-left perforation. (back view - in fluid)

Orange is impossible!!

This is what Mystic Stamp says about the color. But with oxidized colors, it's hard to tell.

Brookman said that the #260 color should be the #229 color. Now for both, that is in shades of orange BUT there is a red orange shade there for #260 besides. #275 is in orange besides red orange but (I believe) that orange is generally a stronger/deeper range of shades than #260. The stamp here is a touch oxidized making it harder to tell. But if the color below the shoulder/base is what the color actually is, I would say this is #275.

As Don said, try the flash test. Also you can try viewing the stamp at an extremely low angle against a strong light to try to see the shadow of the watermark edges.

Soak the stamp to get rid of the hinge and then use Clarity in a glass tray under strong light. A color filter as recommended in this Linn's article may also help: https://www.linns.com/insights/stam...ermark-.html

Also, unwatermarked Scott 260 examples tend to be a paler, more subtle color and often have rough, incompletely punched perforations, also known as "hanging chad".

I too have had issues finding watermarks on some issues. One thing I have done if it isn't plainly evident the first time, try dong it outside on a sunny day. For me, sometimes the natural light, direct or in the shade, have made thing like watermarks or grills stand out more.