CCa "low noise"?

[parrot]

As time passes much of the direct knowledge of valves seems to be following along with it and there's contradictory views amongst experts on all sorts of thing that are fundamental.

I have in front of me a Telefunken spec. sheet for 6992/CCa/E88CC and the same values apply to all of these valves. I don't have spec sheets for other CCa tubes but it seems likely that they are similar in this respect.

From this, it would seem that the only difference between CCa and E88CC/6992 tubes is the matter of "low noise". I've read that the CCa is the top 10% or 15% [depending on source] of the E88CC tubes selected on low noise characteristics and that this was a standard set for their use in telecommunications. Given that the telephone system of the '50s and '60s wasn't exactly what you'd describe as black background or audio quality, I'm rather skeptical that the "low noise" standard has much relevance to audio, particularly since the tube specs seem to be the same [E88CC as compared to CCa]. The key to this might be the nature of the low noise test so my question here is:

Does anyone know [not speculation please] how this process of selection was done?

parrot