Donder, I was going to respond with "where the hell are you getting your numbers from?", but a cup of tea and a lie down later, plus further research indicates we're both correct!
Bulletin 367, issued on 22.12.1983 increased the weights for VH-FNO, FNP, FNQ, FNT, FNU, FNV, FNW, MMO & MMS (i.e. all the surviving -200/-400/-600's). The MBRW became 20410 kgs. I think this brought them up to the -500 figures.
Prior to this, the MBRW was 19731 for all of them (excluding the -500's) was 19,731 kgs.
Cheers!
In my defence, I left the fleet in September 1983.