David quoted Thurneysen on the variation of 's' and 'ss' in OI:
> § 144. They [i.e. geminates; by definition almost all internal "s"s
> of OIr. are geminates etymologically] are most frequently written
> double between stressed short vowel and another vowel, and also in
> final position after a stressed short vowel,...; in later sources
> ss appears less consistently...
> § 145. After stressed long vowels geminates are more commonly written
> single; e.g. ... césad "suffering" oftener than céssad; ...
> § 146. Geminates are also simplified after all unstressed vowels,
> especially in final position... follus "clear" (foluss Sg. 40b14);
> is "is", seldom iss;...
Here is a fairly exhaustive (= exhausting = boring?) list
of the non-initial instances of 's' and 'ss' in our current
version of EN. We agree, I think, that rather than trying
to iron them all out into only 's' or only 'ss', we should
use both, but at least be consistent in the spelling of any
given word, and pay some attention to Thurneysen's general
observations above. The only outright inconsistency I've
noted is "cosin" (§6) vs. "cossin" (§14), but we might want
to reexamine the words in column *s* to see if any of them
would be better written with a double 'ss'.
*s* *ss* *?*
ocus ass f*eis[s]
as·bert nessam (5) geis[s]ib
chois leiss (10,12,13) tres[s]
do·aidbitis cossin (14)
cosin (fer) (6)