Just curious if any of our experts know when the bottom outlet petcock, shown on the right of the attached photo, was introduced and what models it was used on.
The left photo shows the rear outlet version fitted to both of my CT1s, AT1-C and my previous CT3.
My current CT3 came with an aftermarket petcock fitted and this bottom outlet one came with the bike as a spare needing a refurb (now done).
As I said, I'm just curious and will probably replace the aftermarket one with this one.

My suggestion would be 1975. The part number for the bottom outlet petcock starts 498, which is the code for the DT250B of 1975. It looks like there has been a slight change of style as well.

Alan, I think it's used on the previous model DT250A which is '74.

Just had a quick look at the Japanese parts list and the part number is 438-24500-01.
Alan, thanks for looking and putting me onto the right model 

I concur! 

While we are on the subject, did the 74 (175 if wanting to be specific) have a shorter throw lever than the long one your pics are showing?

Mother, according to CMSNL the same lever 214-24524-01 is used on the DT175A