This review states Mullard CV4003.
Hi there. I recently had a Belles 22a and rolled a bunch of tubes through it. At the top of the list of tubes I liked was the Radio Techniques sold by Upscale Audio. It is very extended both in the lower and high frequencies. Next was NOS Siemen's -- not sure which ones, but they came from Upscale Audio, too. They had a luscious mid-range, but I definitely preferred the RT tubes for their extension. I was personally unimpressed with the Mullard tubes you mentioned. I can't fault them, but they was much less musical in my system that the RT and Siemen's, although a lot of folks adore them. I hope this helps. Mark