Cool historical letter!
I assume the misspelling of O'Connor was in the retyping.
The mention of 4831 indicates that some of the load development was done after mid-'52. Prior to that, Hodgdon called the powder 4350 Data powder. The Rifleman staff published a few critical comments on that, evidently pressuring Hodgdon to note it was actually 4831.
There is a lot of similarity to the .257 loads published slightly later ('54) in the Speer #1 manual.
Another of O'Connor's favorite loads published in that era was 39 grs. of 4320 with a 100 gr. Barnes and a graphite wad.
Good shooting!
