From: guto rhys
I agree that it would make sense that �stl-� would conform to the pattern of �str-�. Concerning �stlumm� I may well be getting confused between some ancient form or even a Cornish one. Now that I think of it, modern Breton (at least in the dialect I speak) for �bat� is �logodenn-dall� (blind mouse), whereas interestingly a French bat is bald (chauve-souris).
'stlymun/slymun' (and 'slym'). �-yn� is a dimunitive suffix and generally the epenthetic vowel is lost in polysyllabic words.
Does the above throw any light on the nature of initial 's' in early Brythonic seeing that it is preserved when immediately preceeding 't'? Brittonic 't' is supposed to have been very dental, thus explaining Cornish '-t' > '-s' (cf. Penzance < 'penn sant' (holy head (cf.Holyhead!!)).