[AndyGilham]

For the DC Avominor, there seems to be 3 variants, based on continual serial numbering.
1. DC Avominor (standard)
2. DC Avominor H
3. DC Avominor HR (High Resistance)

Now the standard model went through a few changes, to the point it almost seems like a mark 2, but the serial number are continuous througout these changes.

The straight side variant was early and seems to have gone to stepped sides between September 1935 and January 1936. Although serial number imply there were 22704 instruments made in that period, which seems very high.

The serial numbers of the two instrument that this change happened are
18251-95 (September 1935) - Not Stepped
40955-16 (January 1936) - Stepped

So my feeling is that when the stepped case was introduced, they started the serial numbering off from 40000, rather than continuing. Which goes again my earlier thoughts!

I am still investigating the socket transition!

Andy