Hello Martin
Well, I went and did what I should have done in the first place, which was to go away and do some proper in-depth research using my catalogue collection and the power of the Internet. And I found of course that you are absolutely correct, and that I had jumped to erroneous conclusions. Yes, it seems that neither Bing nor Bassett-Lowke ever made an O Gauge two-speed clockwork mechanism. As far as I can determine, the motor in the 1927 DOY must be B-L's first own design and build clockwork 0-4-0 mech, and as you say is essentially exactly the same as in my 1190 Compound. There are subtle differences, which fooled me, but they are not because it was made by Bing. So the middle control lever (which is for the brake auto-start) is not centred in the cab, but closer to the brake lever than the reversing lever. By the 1930 version, this lever has been moved to the centre. And the coupling rods are unusual, but I think that they are early B-L not Bing. Checking this out, all the unaltered 1927 versions of the DOY I can find have these rods, which change to the plain version later on. And these fancy coupling rods are the same in the 1928 catalogue, when it specifies
"an additional gadget to enable the locomotive to start from the track when special brake rail is used" - hence the centre control in the cab.
So why Trevor Salt should have described this engine as two-speed I don't know. I have messaged him to enquire, but I have had no reply.
The reason why I thought there must be a two-speed O Gauge Bing clockwork mechanism in the first place was because I had previously noticed three lever cab controls on these early 1920s Bing engines. Firstly a LNWR short Precursor 4-4-0 -
View attachment 254416
Where you can see the third lever in the centre of the bunker. This one must presumably be prior to 1923. These were the coupling rods that made me think the DOY mech was Bing in the first instance. Another example is a 4-4-0 Precursor in red LMS livery -
View attachment 254417
This one is post-1923, but perhaps not by much. Neither of the engines appear in the 1928 catalogue. The long Precursors, 4-4-2s like mine, never seem to have three levers, just two (brake and reverser). I had assumed (unwisely) that these centre levers must be for a two-speed control. Which begs the question, what are they for? If they are for an auto-start feature, then it looks as if Bing had this "gadget" well before Bassett-Lowke did in 1928. And if so, why was it never fitted to the long Precursors?
Once again, I seem to end up with more questions than answers. I look forward to your comments . . .
John