This letter makes perfect sense to me if he had sent it as his first reply to a fascist in 1946. Why did he correspond with him over 43 previous letters from 1946 and only in 1962 act as though he had principled objections to corresponding with fascists? The tone is not "this time you've gone too far", or "I have decided we're not getting anywhere", but "We have nothing in common and could never converse". I wonder if he realized it was the same guy, or was submitting this to some public forum.

As I wrote above they did not have a long history of correspondence (previous correspondence was mainly with a Gordon Mosley).

The letter written by Russell was preceded by a letter from Mosley (maybe trying to bait BR) on "the root differences between us" in December 1961 to which BR replied with two letters before Mosley tried to invite BR for a private lunch which prompted the letter of note response. I think this makes perfect sense, he initially engaged intellectually, but when invited to associate privately he strongly refuses.

The long correspondence that you describe (from the 40's to the 60's) was with Gordon Mosley of the BBC, and not with Oswald.

The only letters that Russell personally wrote to Oswald were sent in January 1961.

I was incorrect here. The letters were all from december 61 to jan 62.