Nathan
I've written (emailed) to Keihin in Japan asking for information on the release dates and availability of their flat slide carbies.
I'm not holding my breath waiting though.
Judging from the period photos, and JohnnyO's helpful offering, it seems that the issue is pretty settled.
Please note several things.
1. If you could get a keihin flat slide carby off a 1984 works bike you could ask MA if you can use it.
2. Just because something went through scrutineering at one meeting (or championship) does not mean it can't be protested (within 30 minutes of the close of scrutineering).
3. Also
GCR 16.15.13.1. Acceptable for Pre 85 class are machines and COMPONENTS built up to and including the 1984 model. It is COMPONENTS built up to and including the 1984 model. It could be argued that Model refers to the production bike.
My gut feeling is if keihin had the PJ carbies available for sale to the general public "MA
may approve their use for Pre 85. So get the proof and submit it to MA.
Also this is from the 2014 MOMS so it may have changed.
16.11.1.2 The onus of proof of eligibility shall rest wholly upon the rider or entrant of the machine.
If in doubt
ASK MA.