I nearly have my 68 DT 1 back ready for the road. My question is what is the mark on the transmission dipstick for? I know it says it holds 1000cc's. I put in 500cc's and the oil level is at the top mark. Hopefully pics attached.
Thanks

Did you screw the dipstick in?

To check it you have the bike straight upright and level, put the dipstick in so it is resting on the opening, don't screw it in.

That's how I did it. I believe the dipstick is original. At 500 cc it hits the top mark.

I would drain the oil completely and measure it. Then put 1000 cc (1 Liter) back in.

I have the same problem and find this old thread with no real answer. I’ve drained the transmission oil from my DT250 for a clutch refurbishment. When I refill with fresh oil I get to 400ml and the dipstick (just resting in the hole) shows max. The bike is perfectly level. 
I’m wondering if 700ml of oil hides somewhere in the gearbox refusing to drain out fully. Any ideas?

Just drain it overnight incase the hole that lets it drain from clutch & trans is restricted. I "think" the drain bolt overlaps both side slightly--but been a while. Next day lean it left & right to see if any more.
Probably was ok--what you did--but after filling it takes time to fully level & slow to get to the middle. Just fill it & leave overnight propped up vertical & she'll be good. That's just dipping not screwing in on the dipstick.
Maybe fill on the sidestand "kickstand" if you think it'll overflow, then prop up straight overnight to check later.

Yes, that’s the solution! It seems that it takes an age for newly added oil to find its level through the various nooks and crannies in the gearbox. I left it overnight and the dipstick was now dry. Filled up with remaining oil and will check again tomorrow. (DT250 1976 fitted with later MX engine)

The oil passage where the screwdriver is pointing to is how the oil moves from the right side cover into the crankcase. The other holes have shafts in them. So it will take awhile for the oil level to stabilize.

 
 

Almost all gaskets have a extra material sticking up inside which slows and traps oil even more.  Some aftermarket gaskets are really bad. 

If you follow the Yamaha owner's manual process of warming up the engine a few minutes before checking oil level that works too.

Does that hole have the drain bung getting in the way, or partially getting i the way of oil transfer--without me looking for a case.