Verbal stem alternation (palatalisation)

If I understood Miguel briefing right, the stem alternation in eg
Russian verbs (pr. ind. sg.)

mogú
moz^es^
moz^et

or

pekú
pec^ë´s^
pec^ë´t

came about because of the first palatalisation.

Is any Slavic language stage before the first palatalisation
documented?


I see that Tocharian type II presents (thematic), which have the
usual -o-, -e-, -e-, -o-, -e-, -o- pattern palatalise the final stem
consonant before the -e- form. Is this einzelsprachlich?


Torsten