Purchased NOS, very low usage about 20 hours. I put super glue on the base since it was slightly loose when I bought it.
Review from DG.
Another British tube, this one made in High Wycombe, England around the early 1950s. I've seen it branded as Cossor, Mullard, and GEC/Osram/Marconi/etc. It comes in brown or black base as well. This is a very nice tube overall. Its slightly warmer than the GEC U52, offering a little more bass body and punch and more warmth while maintaining good speed and PRaT. Transparency is very similar, offering liquid silky smooth transients and a grain-free presentation. However, this tube is warmer, not as airy, and the slight extra fullness and more tubey midrange make it a little slower overall. For brighter setups and headphones, this tube offers top-level performance while maintaining clarity and detail. It doesn't veil the midrange or treble like cheaper tubes would (this is very noticeable on headphones like the T1 or the HD 800). This tube is also great for jazz and vocals, very natural timbre and tube warmth. Great bass performance and warmth make this tube a winner for me. Unfortunately, this tube can be hard to find, often selling for $300-450$. It is cheaper than the U52 or WE422A though and might be good for those wanting to use it in a WA5 or WA6-SE. Compared to the newer and much more commonly available Mullard GZ37/CV378 skinny bottle, this tube is much better since it's not as slow and the bass is better textured and cleaner sounding as well as being faster and more resolving overall.