Correct... I bought in Germany and imported it into Portugal. The Portuguese only sold the DT125MX version...

I bought it restored Germany and imported it into Portugal. Unfortunately I have no idea how it was restored :-)
I really want to thank you for tying to help.... it's pretty amazing!!!!!!

If your bike has the right side airbox as shown in the parts diagram and you are open to buying from the USA, there are two models we had which came with the early "non-emissions" carb

The US model 1978 DT175E had a "2A700" carb which was the same "non-emissions" design as the early UK carb with very similar jetting.

The US 1979 MX175F, 1980 MX175G, and 1981 MX175H models came with a "3M2" carb which again is the "early design" and would be a good choice for replacement. 

(Note that the "MX" does hot mean the same thing in the USA as it did in Europe.  The 1979-81 MX175 we got here was a stripped down (no lights) trail bike for off-road use only and did not need to meet the same emissions requirements as the street legal DT175 models, engine was identical and the MX175 came with the earlier right side airbox)

Thanks again! Will do. Wow, what valuable info! Will do...
Cheers,
Markus

Here are the jet specs for the US model carbs.  They are the same carb body type as the one in the 2K4 manual posted.  (Later US models had a carb body with different needle jet design which seems to be what your UK expert said about the 3U5)

2A7 00
main jet 140
pilot jet 22.5
needle jet O-2
needle 4J13
throttle slide cutaway 2.5

3M2 00
main jet 130
pilot jet 27.5
needle jet O-2
needle 4L6
throttle slide cutaway 3.0


I've run both of the above carbs and like the 3M2 the best.  It's just slightly richer than the 2A7.  The later US model carbs were almost too lean. 

2A7 jet specs are virtually identical to the 2K5 specs you posted with a small difference in "idle" and "just off idle" calibration...  (USA 2A7 has a slide and pilot jet that are both one step leaner than the 2H5 specs you posted.)

Lets not get ahead of ourselves here. Euro 2k4 for 78/79 specs are:

2H5 carburettor
main jet 130
pilot jet 25
needle jet O-2
needle 4J13
throttle slide cutaway 2.0

OP bike is almost certainly a late registered DT17E/ F in the US or a 78DT175E/DT175MX in Europe.

If it were an European spec 80/81 the model number would be 4X4.  But confusingly  also be idenrified as as as a 2K4 engine.  But yet also as a 3J1....
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Those are the same specs that were posted by the OP...  he said he hasn't been able to find a 2K4/2H5 carb.  Just posting some options...  or maybe someone in the UK/Europe has or can help find a carb? 

Nice to have confirmation of the specs! 

Had a bit more time to stick my nose in some more paperwork and actually run my peepers over my own bike to correlate with the info I am reading.
  Correct carb for Euro DT175mx 2K4 model could actually be either a 2A7 or 2A750 depending on jetting.  2H5 appears to have the same basic spec..

2A7 = 140 stock main.  
2A750 = 130 stock main. 

My UK carb is a 2A750.

Cross referencing with the European parts book, it appears there is no specific 2A7 variant for a specific european  country.  There is no reference table on carb page specific for UK/Germany/France only parts, unlike some other pages that have differences.

Confusingly the 1978 european DT175MX model code is 2K4 and frame number prefix is 2K4.

The 1979 european DT175MX model code is 2X2 and the frame prefix is....... 2K4.

The OP frame number shows it to be of 1981 vintage.  So it is a 4J4  With a 2K4 prefix..  Number 2619 in the production run. 
The OP 2K4 frame prefix threw me right off the scent.  I can see that the 78 and 79 would be 2K4 as they are essentially the same bike, but the 80 and 81 are different.  To find those still had the 2K4 prefix threw my head into a spin.

It should be fitted with steel square section swingarm usually painted silver, non leading link forks and I believe came in red or white.  As Mark T says, oil tank on right of bike.

I need a lie down with a wet cloth on my forehead... Yamaha model codes are very confusing and often the info in one book conflicts with the info in another..
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Markus, pretty sure you're correct when you said "My Yamaha information lists the bike as a 2K4 model 1978 DT175 MX UK or Germany spec.
The carburettor for this model should be marked 2H5" as that ties in with my info below.
I have reached out to a chap in the UK who knows the DT175MX very well and also sells a few spares, I'll let you know if I get a response.
The information on alternatives Mark gave you will hopefully help.
I think you already know the important information on the bike is the VIN showing it to be a 1978 model, it being registered as 1981 won't make any difference.
I have a 1973 CT3 which the PO registered as a 1972 because the bike has a manufacturing date of 1972 and that was the date he put on his paperwork.
Best of luck.
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Take a look at 4J4 on your list,,,,

I fell into the same trap and had to do some hurried editing.......