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Thanks again for your most helpful input and comments. The machine was inspected over the weekend and so it rests with our friend the marque specialist at VMCC.

I will follow up your suggestion of referring to the VMCC Register - something to do on one of our wet winter days!

As you say, the Excelsior catalogues give useful indicators as to what was being offered by them each year. The 1929 and 1930 cats clearly show the 350cc OHV bikes. I do not have the 1934 catalogue so your observation on the absence of 350 OHV in that edition is useful additional ammo.

That's pretty shabby not getting a response from the specialist. How long ago was this? You could alway repeat andI could maybe get some UK stamps over to you?

Thanks again and I'll let you know what comes back to me on the Excelsior from VMCC.

Thanks John, reassuring. My concern was whether the Y refers to twin port (which it is) or year. The 'S' certainly fits with everything else about the machine and the date of original registration.

Can anyone help decode the following JAP engine serial number?

I O Y/S  54073 /J

I have interpreted this as

I = 350cc
O = OHV
Y = Twin port
S = 1929 build
54073 = Serial number
J = non standard

Am I right and in particular the year number?

Thanks

Paul

Thanks for the comments. The exposed electrical wiring is entirely my fault. I totally forgot (old age?) to put the battery box in place (with the terminals then hidden) before taking the pictures. The saddle does look odd but they are the original springs and the saddle itself is an exact replica of the original. Its position is very close to that shown in the 'before' picture. As I am not likely to be riding this machine, it's far too heavy for an oldie like me, I will leave it for next user to adjust to suit.
There are plenty of small 'tweaks' still to do on this and it is the first such job I have ever undertaken. So it is helpful to have comments from enthusiasts who can spot where things are wrong!

Thanks Ian and Leon. Your help and advice during the process was essential and much appreciated. Paul

Thanks Leon & John, I'll follow those leads 

Hi John & Ian. Thanks for your replies - John for the Rudge diagram and Ian for some encouragement to persevere with the 3 brush setup (likewise, I will never be out on the roads at night!).
Looking at the Rudge diagram I noticed that the resistor was bridging terms 2 and 4. Looking at the inside of my Miller I see there is a bridge (photo on my post a couple of days ago shows this) so I have suddenly (and belatedly) realised this could be a resistance - even though it just looks like a copper/brass connector, So I dismantled it and checked with the multimeter and low and behold it came up with 8 ohms!
The puzzle now is why is there a second resistor (also measured at 8 ohms) between 3&6?
I'm so sorry to have misled you chaps by saying there was no resistance - anyway, I guess this is good news.

Hello All, afraid I’m chasing for more help on the 1929/30 Excelsior electrics and in particular the wiring for the Miller headlamp (unfortunately the original wiring had gone by the time I got to work on the restoration so I have little to go on). To recap, the machine has a BTH mag/dyno feeding into a cutout coil. Standard 6v -ve earth; tail light, hooter, ammeter, headlight with 2 filament main beam and pilot light.
The accompanying photos show the headlight terminals numbered 1-8 (anticlockwise from the driver’s view). Numbers 2, 6 & 7 are wired terminals. The rotor has 4 positions (again, anticlockwise from drivers view) Off, C(harge), L(ow), H(igh).
I have worked out the bridging of the contacts as follows for each of these positions (the first column relates to the first bridging contact going anticlockwise from the control lever, the second to the second bridging contact)

Off      4+5      1
C      5+6      1+2
L      5+6+7      1+2+3
H      6+7      2+3+4

Any thoughts/ideas greatly appreciated!
Paul.