Maurice Forissier states that the Verney Carron, "acier diamant" was in fact provided by Holtzer.
I think that I read somewhere that at least in the 1980s, VC was one of the largest tube maker in Europe. They have had a cold hammering tube operation for a long time (I don't know how long).

There were quite a few barrel makers in the St Etienne area. Ronchard-Cizeron, Breuil (who also used Holtzer steel), Heurtier, etc... It does not mean that V-C did not do it either.

Best regards,
WC-