[turretslug]

I suppose the time-span of GZ34 manufacture represented an era of tightening tolerances and repeatability and improving understanding of things like cathode materials/preparation, so incremental improvement was possible with little or no expense in unit cost, so no perceived need to trumpet it.

Also, the PCL85 had become notorious, so a definite statement of improvement was needed- or "re-brand" as we hear nowadays- to re-assure users. Presumably, the GZ34 was seen as a good 'un from the outset- (slightly) lower cathode heating power than the established 5Z4 workhorse but improved ratings all round, so no need for change of identity as it got better over a few years- when folk trust something, they like stability.. Indeed, an extra 10-20V of HT with later versions might not have been shouted about in case it worried nervous consumers. Most of the time, it would have been fine- slightly hotter output valves, a bit more undistorted output power. Shunt voltage regulators could have been a little over-run, but perhaps we're now getting into expected component lifetimes versus consumer turnover rates and the stats said "not an issue".