Hi, I know not so retro, but will rear mechs badged as 10 speed be ok over 9 speed.
Been using 5, 6, 7 & 8 gears over the years and apart from my 7400 Dura Ace rear mech being a bit of a case by itself, I've found it was the lever that determined the movement of the rear mech.
I'm asssuming a 10 speed rear mech (say 105) will work fine and dandy with a 9 speed STI shifter.
All advice gladly accepted.
do not despise the snake for having no horns, for who is to say it will not become a dragon?