So I have a 1975 dt400 thats on a 1975 dt250 frame. The only thing thats keeping me from riding it is the rear sprocket is bent. I have ordered a rear sprocket for both the 1975 250 and the 400 and neither fit. So It seems this rear wheel is also off of another bike. Here are pictures of the sprocket and casting marks inside the hub. They read '1 up top and 11 on the bottom. Note that I seem to have the stock drum brake assembly thats on the 400. There is no cush drive in the hub. The sprocket is a 51 tooth sprocket. if that helps.

   

 

If it is a Yamaha part, I would expect to see a three character ref, possibly followed by a two character one, cast into the wheel hub and the sprocket mount - eg 168 00.

Deja Vu on this thread...  seems familiar.

My guess is Yamaha 1972 to 1974 MX 250/360/500 hub...  a picture showing the area of center of hub between spokes might help

Goofed looking at Partzilla and very tired..  72 RT2MX  73-74 MX250/360  SC500

Might find a "322" cast into it.

Cast into what?

Hub

And the inside of the brake plate. 

As we can't post photos at the moment, type Yamaha 25120 (the part ref for brake plates) into fleabay, pick any new one to look at (new, as the markings will be clearer) and you will see the numbers cast into the inside.

To look at hubs, type Yamaha 25111 into fleabay and repeat the above.

Sprocket carrier looks to be on a separate bearing & probably a very short stub & axle passes through the lot. Maybe that's a clue as to the model of wheel. Early TT & XT 250's had that but brake plate slotted onto an anchor where i can see this one through the other side has an arm lug. Looking again it's not a floating brake plate as looks to have a bolt sitting there like a solid arm not floating. Might be wrong.

As for the hub that mounts the sprocket, my bet is with Mark: "72 RT2MX  73-74 MX250/360  SC500 Might find a "322" cast into it."

I think you need a 322-25451-00, 72 RT2MX, 73-74 MX250/360, or  SC500 compatible sprocket.  That part number is for a 51 tooth.

Search for a rear sprocket for those years and models.  Parts unlimited lists a  CT243-51S for the MX360.

You could also find a stock wheel and then you would have more sprocket size choices.
 

Same exact casting mark was found inside of my Suzuki ts185 rear hub below "Made in Նapan"

Edit: it was the front hub