I have edited corrections into my post above, as I now believe the extension tubes have 'VR' designations, in the same way as the VR1 and VR2.
The confusion arose from some very old BBC literature that I have, which refers to the tubes as C29 and C30, and which I now believe to be inaccurate, as one of my short extension cases clearly has VR29 printed on it.
The extensions do, in fact, have C29A and C30A engraved on them, because the designation of the entire assembly changes
as soon as the extension tube is added.
You can see now why there is so much confusion around these microphones .... I didn't mean to add to it!
Dan