On the online parts sites (Partzilla and CMSNL), all of the AT1 and CT1 models are shown as having the two rear engine mount plates that bolt on to the frame, two bolts each side. However, my 1971 CT1C does not have these and when I looked at frames for sale on fleabay, only a frame from a 1969 CT1 had the mounting holes for them, all of the others didn't.

Thanks.

Alan, both my CT1-B and CT1-C have the plates, my CT3 doesn't.
What's your VIN?

My CT1’s all have the plates too

Thanks chaps.

Photos of what I believe to be the relevant area attached, plus the frame number.

Hopefully I'm not having a big senior moment!

    

Looks like a CT2 frame..  Yamaha may have updated at the end of the CT1c model run.  I've seen a few undocumented changes but that's a pretty big one.  . 

Does it still have the sticker on the steering head?  It's possible the original frame was damaged at some point and needed to be replaced.  Dealer or distributor may have stamped number into new frame. 

How does your exhaust mount?  To the rear shock?  Do you have a hole in the frame bracket under the seat right by the rear seat hinge to mount the pipe like the CT2/3 frame?  If you do, it may be the original frame was replaced?


P.S.  On the High Beam indicator I found a bulletin dated 12/71 that they went to the new plastic style.  So your late production CT1c probably has the plastic type.

Thanks Mark, great info.

Exhaust (currently Bassani, soon to be swapped to original) mounts on the shock.

No hole for an alternative mount, CT2 style, that I can see.

Sticker still present and a manufacture date of 4/71 seems, to me, quite late for a 1971 model bike.

Relevant photos attached.

     

That was probably the last or next to last month of production.  Looks like they might have changed the frame design sometime during 1971 production. 

If it was a "replacement" frame from the CT2/3 it would likely have the exhaust mount hole bracket across the frame instead of the one you have.  The bike I know of from decades ago that the dealer replaced the frame on the original number was stamped in the new frame but the dealer said Yamaha could not provide a replacement decal.  He had some paperwork from the dealer that certified the original frame was destroyed which I guess made it legal to stamp the new frame with the same number.. 

Thanks Mark.

So I have a part factory, part home made (AT engine cases) "mongrel"!   One day I WILL buy a bog standard, matching numbers bike! 

I know what you mean about frame replacements. My 1976 RD400 had a replacement frame around 1978. Unfortunately, in Europe, the frame changed quite a lot between those two years (I think that the US kept the same frame, at least until the Daytona). 

Hi .How's the headlight fix going .
regards keith

Headlight arrived and looks in good shape.

I'll email/message you in the next day or so.