Thanks guys.
Kevin,
IIRC, the .357 didn't come out until the second or third year of production.
There was a gunsmith back then, Rick Devoid?, who was reaming the .38s to .357 Mag (limited to 125 grain bullets (for their shorter OAL) due to short cylinder) and calling it the Pocket Rocket.
Ruger introduced the .357 in the SP101 after Devoid's conversions proved durable and safe... but the first factory .357s were limited to 125 grain bullets for the same reason as the conversions (short cylinder).
Later, Ruger lengthened the frame window and cylinder so the .357 could accept any bullet weight.