1880 Washington DC Congress Cancelled Cover & Dnc Letter

Knew I had this letter but just found I had an old image of the cover too. Headed to StampSmarter to try and figure out exactly which Washington 3c that is... Pretty interesting letter from the DNC to an old General to boot. Plenty of info on him out there.

Will scan the stamp and upload close view later today. I feel a bit smarter... and confusededer..
It has the secret mark (so Scott 158-184-207 are possible), and I believe it is from the original engraving (limiting it to Scott 158 or 184- I think). On cover I am struggling to figure out which paper it is on- I am leaning towards thinking it is #184. Any suggestions on how to narrow this any further without soaking it off?

Was hoping the SS page would end the #169/#194 note with "collectors and also given to members of Congress" but no....

This is a good example of stamps vs postal history. This may sound odd and I will likely get criticism for it, but the ID of the stamp often doesn't matter. Such is this case. Even though the cover is a bit doggy, the address and the contents drive the interest and the value above whatever the ID is. Soaking the stamp will only ruin a cover and destroy value for no real end.

(207 wasn't issued yet, so it can be eliminated from consideration.)

Hi John, I could not agree stronger. I am here for the History. Unused stamps, and coins, just don't particularly interest me any more than air filled or rolled beer cans. It means they never served their intended purpose. And please, before I go goring anyone's anything, I freely acknowledge this is my view of things and I certainly understand condition condition condition and that the details of manufacture are important history as well. Enough of that.... Here are some closer views...

It's a 184.
"Secret Marks", is from 1880 (so can't be the 207 which also has a long _ recut under the S of Cents), and it's on soft paper (as evidenced by the lower right perfs).
It's only other possibility for the design type is a 158, and there is a SLIM chance, it could be on Double Paper (158a), but that is reasonably unlikely, but I can't tell that without having it in hand.

There is a lot of info on Gen. Slack, but came across this sketch attributed to him from the Battle of Champion Hill where he served as Colonel under the command of US Grant. This was during the Vicksburg Campaign. Maps show Slack's units were right in the thick of it.

http://www.mkwe.com/ohio/pages/H017-17.htm

James Richard Slack's bio from Appleton's Cyclopedia of American Biography, 1888. Vol V, p546.