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.

   

That casting mark is not useful for identification.  Should be something like at least three numbers or (usually) more in a line.  It's possible it doesn't have one...

Does it have a cush drive under the sprocket?  That part should have a number as well...  on the underside.