what was the earliest dip switch?

[JonE]

Thanks both - that clarifies. So I'm imagining (as my situation is same as Jim's) that the SM5 switchpanel essentially remained unchanged as the column mounted switch came in. See Dave Mann's post in other thread Jim and see if you concur with his points?

I'm now intrigued about R70s - are they just after R47s, and steel rather than brass but otherwise similar but with a slightly different bayonet? I was thinking March 1929 would have had R47s.