I bought 25 BC239Cs from Langrex. The theory is that Naim selected these for Vec (since the BC239 datasheet only claims 25V), and then for matching.
So I avalanche tested mine with a 0 to +86V supply through a 33K resistor to the collector, and the emitter to ground.
With the base also tied to ground, they all held tight all the way up to 86V.
With the base floating, about 1/3 avalanched just above 50V, 1/3 avalanched between 50V and 60V, and 1/3 had not yet avalanched at 61V (I didn't run them all the way up to 86V in this test).
So I don't know that there was a lot of selecting to be done....