Is there a way to recover a Steam code if your missing one of it's digits?

I'm afraid there is no recovery for that. You can at least try to bruteforce the missing character.
No worries, there is no punishment for too many activation attempts, only a cooldown. ;)

Yeah after 10 unsuccessful attempts you'll be unable to activate any key for 1h. So for just one missing character, I guess you can bruteforce it in 3 hours, if you're unlucky

My copy of LSL6, which was also from Tremor, but a few months ago, has 8 as the first and 5 as last.

EDIT:
I also got the rest of the LSL games at the time and the first character for all of them, except one, was 8. The one exception was a 7.

I still have mine in activation history so maybe this can help you somewhat

CYNK0-*****-FFDYK | Status: OK/NoDetail | Items: [226444, Leisure Suit Larry 6 - Shape Up Or Slip Out]

There were at least 2 different bundles so possibly two different sets of keys out there.

Assuming the key was copy-pasted, it may be the first character which is missing. Perhaps someone accidentally started copying at position 2 ?

First five in my larry 6 code is TJ84K
So yeah don't forget it's not just numbers, but letters as well. :D Would probably be best to just try them in order since it will take a while. 0-9A-Z first position, then second, then third, then fourth, then fifth...