Ask Andreas Wiemer for his opinion. Post a comment on his profile:
could you post a scan of the whole letter?
Here is the best scan I can do, hope it helps! As you can see, the letter is to a member of the House of Representatives and the man must have been a friend of John. I felt due to the content of the letter, this is one of the few that Kennedy personally responded to at the time. The signature looks to have been signed super fast, and I've never seen just a Jack secretarial besides his early secretary from 1949-1952. Hope I got a good one, as I always admired Kennedy.
I think its real. You are right, authentic signed letters at this time are rare. "Jack" secretary signatures are not unusual and you can find some later than 1949-1952 signed by Evelyn Lincoln, but she has her job not really well done. ;-)
Hope thats of help.