1870 Cover + Eku Question

Here's another cover to the same family as before, this time from May, 1870.
The stamp is a 136, meaning it was issued a couple of months before the letter was written.

My question is twofold. 1) Are there any sites/catalogues that keep an updated census and record of EKUs for each classic stamp? I have a few I want to check, but this one as well.
2) Is there ever a time when the letter within the cover adds validation for the time of posting?
I ask because the year can not be seen clearly on the postmark.

Updated Earliest Documented Use information is hosted on Richard Frajola's webpage - https://www.rfrajola.com/mercury/SiskinBerkun.pdf

Letters get separated from the envelopes and sometimes wind-up being put back in the wrong envelope. Contents can be used to date an envelope, but typically are not sufficient to prove an earliest known use.

The APEX maintains such an EKU list as well for the specific purpose of certifying EDU or EKU cert requests. Scott's US SPecialized Catalogue lists many such dates as well.

As to dating, there are also known periods of use for certain postmarks, especially nice ones like the one you show.

Lastly your letter seems to be correct as the "may" matches as the letter's "90" helps decipher the cancellation year shown.

The fancy cancel certainly has some eye appeal

That's an 1870 cancel, not 1890. And it's clearly grilled, so 136 it is.

SPQR

The list you provided on Frajole's page is wonderful. Not only does it catalogue the Scott numbers in order and provide the certification agency involved, but even lists EKCs of varieties such Double Paper.
It is very impressive! Saving this page.

Thank you for all the comments.

My initial instinct was that it was a "7" rather than a "9". But maybe that's because I have a number of letters written by the mother and sister of Mrs. Sarah Adams (formerly Sarah Sampson), and their 9's tend to put more emphasis on the curves.
I've had a lot of fun studying them and trying to understand them accurately.

It's also neat how some letters of the 1800's only have the name of the person and the town for the postal carrier to bring them to. In those times, there was more importance placed on a name rather than a number.

In most of those small towns, there was no carrier delivery. Residents would go to the post office and ask for their mail.