Rivets
I have never been aware of any version of the AVO8 that measured true RMS. I have just checked the schematic of the AVO8 Mk 5 (original and later versions) and it is simply a dumb old average-reading RMS-calibrated moving coil meter. The RMS calibration is true only for sinewaves and if I recall correctly the fudge factor is 1.11
True RMS meters belong to a later era, the first I am aware of was made by Advance (later Gould) at Hainault during the early seventies and rebranded in Germany and the USA.
In audio work during the seventies we generally used quasi-RMS circuits which read a combination of average and peak. I used the Analogue Devices True RMS IC in audio instrumentation designs from the mid-seventies onward.