My bike is a ‘73 DT3 250. It took quite a bit of searching but finally found what the second bolt on the bottom of the engine is. The Service Manual refers to it as the change lever guide or neutral stopper. Mine seems to be leaking oil and I don’t think I should try tightening it any more. Is there a crush washer or gasket under it?  Is there anything in particular to do or watch for in taking it out?
Thanks for any advice!

Oil will drain.  Should be a spring and plunger underneath.  Sometimes missing if someone removed to drain trans and didn't notice spring and/or plunger falling out.

If yours are missing, replace them.

Refer to parts diagram "Shifter A" which also shows that yes, there is a sealing washer.. 

Thanks Mark! I couldn’t figure out why it wasn’t shown in the Crankcase area for parts, thanks for pointing out Shifter A…

You're welcome.  Just to clarify, you do not need to remove stopper bolt to drain trans....  often it is mistaken for the drain plug and often the spring and plunger are lost when that happens. 

Bike may or may not shift correctly with those parts missing.  Sometimes they shift normally with spring/plunger missing and then start jumping out of gear or hitting false neutrals when shifting after some time goes by. 

Appreciate the additional info. From what you said earlier, I was wondering if the trans would work properly without the spring and plunger. I haven’t had any shifting issues…yet. Guessing previous owner did use it to change oil and didn’t replace the gasket. I’d rather leave it alone, but want to stop the oil leak.

Also, I see the drain bolt and neutral stop bolt gaskets are different part numbers, so assuming that means different sizes. I guess I’ll order one of each…