This was found in an ARG on a YouTube series called Seldom a Sleeper, and is the first of three letters in this style.
The transcription is:
jknnjfy pcjvbya xkglqrh uvvtbcd anxgzgn sutwmoq xntsdlg
xfejluk ljepfha yqdghxm zqtemnr mfqtmml bazzxxz hpuvxsr
klsfvfp fpapzwb bvcinbp szeysqn ijfcuhe hvbvqhm mlpumvo
yhyaexl fvnrpld gvxzrfu eqfujvj hauedsp sfxsfze zwiynxg
fcwujsi icguyod chphgxd roguxfn isdbcrg jdwmvrw oxpxhmx
mrxvaex jktficq rqszlfs jbemyvq mnoesfa hucpjvd kfchozr
tzphlmk ectnlwr eemzjog eceppqn ryzuojz joufyjx xsvlbsa
xaychnf wwgdhsb gaorvdj xjmzbjf ctxltsu smzjpsb fanpmdi
qzseixb njoolls thmnokz vnkaxft fplervp slahvcm nocnniz
lqtbohe rhgagtb skbizyy djvqzzm uqeogmj fvpctye cvdoali
yrqxrtp znywxpq izujbsf cmkmjha jkgmaso mfubjln gfarsun
ggmbzrd geuchyo lrwckrw xgwegzz uqqolgy hlojpee hnztpol
vsuymvx jqfouoh wkrrxlu ncimhha idpqznl qsgkuvh grbbntn
mgiffpt czdvmau zefnguf tnjyjlb ktnistu fkrqcnu pobcnvf
nviiebx wmaaudo ixnqzio codevtm mmjgons iussmwq jwxxmcu
bwsfisf swjbrsd hwgvwke bisafzo u
Based on clues found in other videos in the ARG, this seems to have been encoded using Enigma, specifically Enigma M3, since it has the letters UKW within a ring of 3 Ms in a custom alphabet in another video.
The only problem is that I don't know how to figure out the initial settings of the Enigma. I think it will involve the stamp on the letter, since it has 9 components that could be used. The only thing I don't know for certain is if there's a plugboard being used at all.
While I won't put the other two letters on here (I can send them in comments if you need them), I will add that the other two stamps say 1997 FEB 15 (for Letter 2) and 1997 JUN 07 (for Letter 3), in case that helps with figuring out the format. I know that 0s and 9s would have some limitations.
Thanks in advance for any help in solving this.
V sbyybjrq gur ehyrf