[pmmunro]

The Lucas connection is certainly possible. Although a bit tight, the 15V "overvoltage" range would have worked for nominal 12V motor vehicle systems which were designed to charge at 13.6 to 14.4V.

It might be thought that the Model 'D' would have been ideal for the application, or at least a modification of it. They were widely available on the surplus market during the 1960s, which is probably why they are still quite common. However, in 1947 there may still have been some contractual inhibition to making an Air Ministry specification meter available to a commercial customer. Even if there had been, simply using the normal Rev. MC button rather than the K=1/K=2 switch would have been a significant distinction.

There is the urban legend of ministry surplus Avometers being dumped down a disused mineshaft but I've never seen or heard any firm evidence to support this. The possible benificiary would have been AWEECO who would not have wanted to lose sales to surplus units, but given their chronic inability to keep up with orders until the Dover factory was built, this seems to have been more an anticipated than a real problem.

In support of the possible Lucas connection, there is the Model 12 which was designed for servicing automotive systems when alternators first came into common use. I understand this design was instigated by Lucas and there are Lucas doides in at least some of them. The choice of these diodes may not have had any link to the Lucas involvement in the design.

PMM