Thanks!! I was pretty baffled. I wonder why they spelled it that odd way; it seems to break the rules for Latin phonetics to ram together three consonants like that.
Unlike all those Slavic tongue twisters resulting from vowel loss,
stl- is at first blush no different from
str-. But somehow both Latin and Germanic allow the latter while disallowing the former. In Slavic
stl- also seems to only result from vowel loss, so the reason for the difference could be that
str- was much more frequent in Proto Indo European. Many languages don't distinguish between L and R, and
str- is clearly the easier one to pronounce - just flick your tongue - so maybe this difference in distribution goes way back. Still, from PIE down to Latin
stl- was evidently allowed and maybe not even that rare. They must have started having trouble with it after all the consonant assimilations forced
-tl- to be replaced with
-cl- inside the word so as not to end up assimilated to
-ll-; word-initially it became a single
l- (tollere > lātus). The initial
s- probably prevented this in
stl- at least in some varieties, and there was also
scl- as a non-standard treatment.