I figured I’d chronicle my findings in case another forum member has similar circumstances. 

I purchased a fully restored US spec 1981 DT175H this summer. I got it in California and brought it to my home at 5000 feet elevation in the Rocky Mountains. After a proper break-in due to a fresh rebuild, I addressed the jetting. 

Due to EPA regulations, the US spec DT175H carb (3J0) has a tamper proof brass cap over the air jet and a jet needle that has only one notch and is not adjustable. The needle jet is also an oddball series which can’t be replaced and you are stuck with. The air at 5000’ is a lot thinner than sea level and creates a rich condition. Because I was unable to lower the needle to lean out mid-throttle, I tossed the carb into the parts bin and replaced it with a carb off of a US spec 1978 DT175E (2A7).

The factory jetting in the 2A7 carb is:
Main jet - 140 
Pilot jet - 22.5
Jet needle - 4J13 clip position 3
Needle jet - 249 series O2 

Since I ride it at high elevation (5000’-8000’) where conditions are rich, I figured I’d start with the factory jetting for the 2A7 carb listed above. I quickly discovered all circuits were running lean. I was confused so I did some research and learned that model years 1980 and 1981 have a larger, more free flowing airbox compared to earlier models. This increased air flow explains why the jetting was lean. 

After numerous adjustments, I settled on the following:

2A7 carburetor on a 3J0 DT175
Main jet - 155
Pilot jet - 25
Jet needle - 4F15 clip pos 5
Needle jet - 249 series O2

Plug chops show good color and it revs freely to redline in all gears (7500 in 6th). Hopefully this data might assist someone else trying to rejet a 1980-1981 DT175 with a carb from an earlier model bike. 
 

Another option in the USA is a 1990's RT180 carb on the DT175, often easier to find than the one-year only 1978 carb as 79-81 had the emissions restricted "tamper proof" versions mandated by the US EPA at the time.