The DT250/400 Mono shock, went right up thru frame ,and bolted at the front of the tank,. is the Xt the same,? lots of them about ,[ i might have one in shed, ,what beats me is the XT500 REMAINED A TWINSHOCK,
You"l need to fill in V55 form, [ print it off from that Gov,uk site] ,M,O,T on the chassis number, Insurance on the chassis nmbr, you can get a dating cert from Air-cooled RD forum,[ need to join, £ ] , unless you have the original TITLE, [usa log book], ,And that Nova , he mentioned ,?
Welcome in, if there stiff ,thers too much oil in, it will blow the seals again , glad you removed the extra 100ml , as the DT250/400 is only 185ml, and are much larger,,,?
You see the pawl [ claw] it rotates the drum [ selecting gears,] but there is another small spring on a small detent arm, THIS HOLDS THE DRUM FROM FREE TURNING] AS YOU SAY FALL OUT OF GEAR, . look for it, it be behing clutch basket, was there any metal parts in the oil or casing when removed, ? do you have a manual ,
Unusual" ,most start with a number [ 455- 000-000] ,what country you in ,is it old or a modern DT175 [ Far east]
https://www.motorcyclespareparts.eu/en/yamaha-parts/yamaha-model-prefix-motorcycles-1970-1994