Thanks Mark.  Found a link to the post. 
yamaha-enduros.com/index.php/forums/74-7...sion-bearing?start=0
 

That thread never really solved the issue, just identified it. 

Taking a look at the old printed parts lists (some are in the Tech Library) reveals the correct part number that I showed above.  And it is still available.

(It appears to me this error has nothing to do with superseded parts numbers...  what appears to have happened is someone at Yamaha typo'd the original part number -30517 as -30512.  Then the -30512 number was superseded but it wasn't the right number in the first place!)

So just buy the -30517 bearing and you'll be fine.  I linked Partzilla's ebay auction above or they should have it in the store.

MarkT nailed it!!.
Looks a pretty trick roller bearing & out of curiosity never having seen one i see TZ's run one both ends of the crank.
So i'm guessing the twin crank must be thrusted by the ball race bearing in the center two bearing.
But then again i can rubbish my theory as only see retainers under the outer two trick bearings.
Are they rollers that take side thrust MarkT?
www.partzilla.com/product/yamaha/93390-00012-00
Anyway--i see on the partzilla link to 'that bearing' if you pull the list down it says DT250A & 360A at the bottom--which we know is wrong!!.
Makes life difficult when they get lists wrong.
Could grind the pimple off if dimensions are the same & if its designed for side thrust.
Ok forget that, or tell me off, it's something i did once when too stingy to go buy the correct one for whatever i was building.
Think was a final drive bearing with snap-ring on my RT2 & i had an RD350 final drive bearing so just removed pimple.
Sorry, long spiel about nothing as usual.
I'm editing--i see the DT360A does show this number on final drive bearing. That'll confuse people. #31
www.partzilla.com/catalog/yamaha/motorcy.../dt360a/transmission
TZ crank link below--sidetracking the post sorry.
www.partzilla.com/catalog/yamaha/motorcy...4/tz350/crank-piston
Ok ignore me, i was thinking you were talking DT360A crank bearings but talking Trans bearings i guess.

I ordered that one for my DT 360:
9330420501: Brg,r-b 4205 52mm 185g Ky Yamaha - buy the 93304-20501-00 at CMSNL

Gerry’z; note I am referencing The L crankcase bearing. 20501 is in the R crankcase.

Morley . TZ 's have straight cut primary gears , so don't have side thrust issues associated with helical cut primaries .

There I use this one:
www.woembi.de/Kugellager-6305NR-Premiumlager
 

Thanks Swoop, yeah that fact crossed my mind but the parts drawing shows helical teeth but hey i felt sure they 'would have' had straight cut being a racehorse--so thanks.
I do see the center normal ball race bearings shown in the link have the half circle thingies to keep the crank in the center. Cheers.

I'm wrong there Swoop it's the outer end bearings that have the retainers, so one is 'that roller bearing' on the primary side--interesting.
All have pimples to stop turning, not that they should if clamped tight between cases.

GerryZ- The bearing link you posted is for a 6305 with snap ring designation...  it "might" work just fine.

The original bearing had an additional "X1" designation which has a "special radius" on the bearing.  Picture below shows the radius marked with a red arrow.



Without that special radius, the bearing might not press into the case all the way.  It's nearly impossible to see but the snap ring won't be against the case.  This shifts the shaft slightly to the right...  sometimes enough that the gear will rub against the right case.