If the piston was the MX version , it will only have one ring . A peek down the
exhaust port will verify if that's the case . The carb usually has a stamping ,
usually around the choke plunger tunnel . Those numbers will verify it's origin .

It’s a 2 ring piston, so I’m assuming a standard DT.
The head has the fin profiles for the exhaust, so again I’m pretty sure standard DT. Looks like the only change is the barrel itself. I’m trying to revert back to the standard airbox, so hopefully the positioning will be the same.

look on the intake side of the piston, a MX400B has a lot less intake skirt than a DT400 piston. you can remove the reed valve assembly to see it.