I have a 1973 RT3 but the engine cases is marked (RT1). Is it supposed to be marked RT3 or is there some out there with the same RT1 marking?? Thanks!

They use RT1 prefix right through, but the numbers determine the year up to "73.
One thing [that might not 'be a thing'] is--i know the early DT1 had a 4th 5th selector wrapped around the drum, therefore the 'very early' cases never had a casting hole for the selector that runs on that shaft in later models dt2 dt3.
If i look at the RT1 drawing it shows 'that' in a separate caption.
Right confusing so just saying be careful if buying old cases.
"I think i'm wrong" & all the 360's had the later setup as 360's started from "70 where DT1 were "68 on.
Maybe someone will know for sure [& i should]. Sorry to confuse.
www.partzilla.com/catalog/yamaha/motorcycle/1970/rt1/shifter-1

OK, thanks for the reply. It's supposed to be ALL original that's why I was concerned that it has an RT1 stamping. Thank you.

Frame should have an RT1 prefix as well as the sticker on the steering head as well...  and of course the numbers on the paperwork and engine number should all match the frame number*.

VIN tables can be found in the Tech Library.

* Yamaha has said it's possible the engine number could be a couple of numbers different from the frame number but that's really rare.  Usually the full number before and after the dash matches exactly. 

All good. For some reason i must've thought you were looking at buying cases.
I should learn to read fully haha.
I had an RT3 once, bought as a crash job when i worked for the local yamaha shop & only had 900 miles on it.
Never been off road in that time.
The nearest i'll ever get to a new one that's for sure.