The suggestion that it may have been a striker from a bilingual - cyrillic typewriter that was mixed in is an interesting possibility; someone transcribing diplomatic telegrams in WWII may indeed have need of access to Cyrillic typewriters…
Randomizing/obfuscating the letter case might buy you a little time, though I think it's something else entirely here.
Enaqbzvmvat/boshfpngvat guR yRggRe pnfR zvtug ohl lbh n yvggyR gvzR, gubhtu V guvax vg'f fbzRguvat RyfR RagveRyl uReR.
Some of the E's look a little curly like epsilons but I'm guessing that may be an optical illusion.
But check out the 3 in "chancE3"
1) it's just the typeface,
2) the teletype machine has unique letter so the machine it was received in is known (and hence which staff received it), reducing the ability to forge messages. Different machines could have had special letters, or all machines handling secrets had that particular "e"??
3) the machine broke and the repair shop only had a small-caps "E" handy.