DT2  .My damaged sprocket cover has Made in Japan cast into it but others I've seen are just plain. Looking at the parts lists 68-73 all seem to have the same part number, which years are supposed to be plain and which are cast Made in japan?

I don't have a definite answer but I'll offer what I know. I have twelve 250s and 360s from '68-'73. Some are original and unrestored. Some have been restored. None have the Made In Japan in the casting. I have bought several NOS sprocket covers over the last few years. 2 have Made In Japan in the casting. The parts listings I have show only 1 part number 233-15421-00-00. There are 2 NOS covers on eBay now. Both have Made In Japan in the casting.

So there seems to be no "correct" part!!

Just looked, mine has 214 15421 00 cast inside

There absolutely is a correct original part which I suspect is the one without "Made in Japan" cast in...

(If I remember correctly, it was about 1974 when "Made in Japan" showed up in the cover casting on the new models?)

There are also "replacement" original parts that will function perfectly but might be cosmetically or otherwise different.  This cover is one of them.  Another example is the gauges on the 1968 DT1...  speedo and tach were unique (small tach is very obvious).   For decades now the "replacement" gauges supplied by Yamaha were the same as subsequent year DT1's...  The revised gauges work fine on a 1968 model but they are not "correct" if someone wants an all-original 68.