650 Motocross Bike

Have a look at www.tri-corengland.com which may help you.
I reckon that the 'D' in the number equates to a 1970 frame/engine. As far as I am aware there was not a specific sidecar frame at that time. The bracket under the front of the tank is a mounting bracket for the standard fuel tank and the one at the front of the bottom frame rail would be for the side stand. However, the bracket under the top frame rail above and behind the carb(s) could easily be a throwback from the sidecar days.
'Tis a long time since I owned such a beast - last seen on my '69 Bonneville in 1972 - so if my memory is playing tricks I apologise.
There will, no doubt, be someone along soon who has access to a similar machine who can confirm/refute my ramblings from a time long gone!

 

Your frame is a standard 1963 T120R The 11 is a U, so your number is DU 50104.
The lugs on the headstock are for police bike fairing fitting and appear on most frames as does the lug under the seat. What are the engine numbers? as you have a single carb head as in TR6, a common swap for dirt bikes

 

Also 63 Bonneville, pre 67 frames were also favored by the dirt racers as it had the 65 deg head angle and not the later 62, great looking bike.

 

11=U? Ah, now that explains a lot. No wonder I was a bit unsure about it. I guessed there was something wrong but I just couldn't quite figure it out.

Nice one, Darkman!

 

Would look nice with an original fuel tank

 

Are you going to strip down the engine ?

 

Hey, It's has the has the same center stand as my 64 Franny B scrambler

 

Hi and welcome.