Hi bitseeker, I don't know about the 5335, I presume it does have the dodgy buttons from the look of the front panel, the 5334 has the same spring leaf potential fault as per most of 1980s HP. Mine is OK at present - I suspect it "knows" I have some spare and pre cut springs - so they will never fail!
I don't know why the time-nuts appear to have overlooked the 5334.
The 5335A is 100pS Averaged, 2nS single shot resolution.
The 5334B is 200pS Averaged, 2nS single shot resolution.
Perhaps there is something else I am missing.
If I want to get all time-nutty I do own a 53230 which is 20pS single shot, I got recently the TAPR TICC, a time interval time stamping counter, controlled by an Arduino no less, which is 60pS resolution - and great value performance/$.
Rob