Santa brought me a couple more 4V rectifiers suitable for use with 6AS7s in the OTL. Both of these tubes can provide 250 mA, similar to a GZ37.
An AZ50 manufactured by Mullard in it's Blackburn factory in 1958.
And a Mullard FW4-500, also manufactured in their Blackburn factory, but sometime before 1955. Starting around 1955, Mullard adopted the Philips production code, but prior, Mullard used an in-house production code, which I have yet to learn how to decipher.
Interestingly, this tube is also labeled as an AZ50, but Mullard didn't begin to manufacture the AZ50 until the late 1950's. The AZ50 was originally developed and manufactured in Philip's Eindhoven factory. Sometime in the late 1950's production of the AZ50 ceased at Eindhoven and shifted to Blackburn. I often see FW4-500 labeled as AZ50, so it would seem that these two tubes were considered interchangeable.
My suspicion is that this is a fairly early tube due to the extra top mica spacer intended to keep the internals centered inside the bottle.
Later versions, such as the one below, used a top mica spacer with teeth.