Your 10-6 K frame came out in 1962 and the later models were rated for +P and +P+ ammo.

As for that box of 100 gr. Lehigh ammo, be it made by Underwood or any other maker, that list the MV at 1,300 fps......

I just watched a site that tested that ammo in a S&W K frame 4"
and I don't want to bust your "Bubble" but, the 38 special super loads, crossed his chrony sitting at ten feet at...........

1139 fps.

Lehigh printed data shows True Blue maximum at 7.5 grs.

stay safe.