Thank you, Warden, for posting a scan of the verso after having soaked the stamp.
I count 14 x 17 points in the grill and believe it to be an E grill, Scott 91, given the vertical orientation of the ridges of the points.
Should you still want to have it expertised, the gold standard for US classics is the Philatelic Foundation in New York. https://www.philatelicfoundation.org/
They do take submissions from abroad, so this is an option for you.
However, given how far off-center your stamp is, and what appears to be some staining on the front that may have survived the soaking, it may not be worth it financially, depending on your plans.
So if you want to stay domestic, you could submit to the Royal Philatelic Society. They could do it given their resources.
which has links to the form and fees required.
Hope this helps in thinking about your next steps.