Little Fish On Back Of Queen Isabella II Stamp? And Advice On Paper Type

Today I looked a some of my early Spanish (including some Spanish West Indies stamps with Isabella on them) I was very pleased that watermarks are really much easier to find and I have pretty much IDed most...until I found this one... which looks darker in real life - picture looks lighter.

I tried over and over to find watermark 104 or 105 - but I am not seeing it. What I did find was a little fish on the back which seems to not belong - any idea what that little fish is doing there? The no watermark version would be on White smooth paper - but the paper seems more rough yellowish to me. Last option - maybe a forgery. Any thoughts are welcome.

Thank you.

Fish.
It can be either a collectors "possessive" damage mark.
Or an "expertisers" mark.

https://www.filatelia.fi/experts/namesa.html#f

"possessive" damage mark was my fear - wow..let me check it out. It was funny to find a little fish though

It's original, no watermark.
watermark cross lines:

watermark "lazos":

Cancel carthweel, regards.