As S&W Fan noted, the AHU prefix doesn't appear in the SN listing in SCSW, which, as mentioned, is not unusual. SCSW actually refers to the 3-alpha, 4-number SN range as the "dreaded triple-alpha range." The AH* SNs that are listed for no-dash 686s are all in 1985, so that's probably the likely year for your gun, but with S&W nothing short of a factory letter is certain.
I have a couple of 686s along with a few 586s (I have a soft spot for blued guns) in both 4" and 6" barrel lengths, all no dash models, some with the "M" stamp, some without. Many believe the 686 to be the best revolver ever made, and that's a really nice example that you have.
ETA: I just noticed that your primarily interested in collecting, so I assume you don't plan to carry the gun. If that's the case I definitely wouldn't send it in for the bushing fix and I wouldn't worry about it. Some feel that 686s (and 586s) without the "M" stamp are/will be worth more (some day?). I don't buy that, but for collecting purposes there's really no reason to take the chance that S&W or one of the carriers will mess your gun up somehow along the way.