That's a good idea...I did find some and the prices are pretty high. Yes, it is used to regulate filament current. It was an upgrade from the previous serial number series, I believe. This is a late serial number and has all the bells and whistles. The reg tube is not essential, but enhances accuracy in case of varying line voltage.
If it poops out, I may buy a NOS tube or leave it in for looks, (it makes no visible glow in this application, anyway) and hide a modern regulator under the chassis with the other new parts.