The owners manuals on this forum show the 1985 DT175 has a 6V battery, but the 1985 DT200 has a 12 volt, so it looks like this timeframe is when they switched. Hopefully someone else can validate.

69 AT1 was 12 volt (in the USA)...  but with a (heavy) generator/starter and they require a battery to run.  As were the US model 125 Enduros until 1976. 

Magneto bikes stayed 6v for a long time...  nothing wrong with 6v.  My opinion is 6v was used because it is easier to engineer a simple system with the fixed output magneto and little voltage regulation. 1990 US Model DT50 was water cooled monoshock and still had 6v.  

The big improvement came around 1976 for the magneto 6v magneto bikes when the voltage regulators started to be used.  The monoshock 175's were still 6v and had a much brighter headlight than earlier models.

Many modern 12v magneto bikes have an excess of available output and heavy duty electronics that regulate (waste) the excess power and turn it into heat..  or sometimes they have multiple charging coil systems that are electronically cut in and out based on needs.

Just not getting a consistent spark when I ground the plug, I get a spark now and then. I need to re-check ignition timing and point gap before BTDC 
Its hard to get a feeler gauge to check spark gap. How do I check points using a light or an meter?
I also need to check the coil resistance does anybody have any numbers they should be?

OK I'm rough I know but let's see if it starts. Don't get too sidetracked on perfect timing as won't be far out if you set the points open gap to say 14 max for now. Timing on those is related to points gap as the  back plate is fixed. Don't set the plug too wide say 20 thou max. I can hear it wanting to start from here. Good luck.

Bikingbagga, a while back whilst working on an AT1-E for Agop we were getting one spark then nothing, stop cranking, tried again, one spark then nothing.
Turned out the pivot for the points needed a little bearing oil as it was not closing the points before the cam had come back round to open them again.
Might be worth a look.
I'm currently restoring a '69 CT1 myself.

I was going to suggest cleaning the point contacts by pulling strips of white uncoated business card through them... 

(but I'm pretty sure that suggestion would be met with a "but the points are brand new" objection.  )

Ignition coil
Primary resistance = 11.29K should be 4.9
Secondary resistance =11.28 K should be 11K
I took the coil off anothe CT and the values are as follows:
Primary resistance = 9.3K shoild be 5K
Secondary resistance = 9.3K should be 11K
Bottom line seconday and primary resistance are the same I did remove the plug cap from both coils before I measured

I can look at the points again but Ricks Electric fitted brand new points and condensor when they rebuiklt my stator 

Primary resistance on the plug coil should be small like under 2 ohms "that's two ohms" not up in the thousands. Secondary should be high & extra high with a resistance plug cap if ya measuring it through there. I suspect if it sparks then it's OK. Can you try the coil off the other bike to observe spark to see what changes if anything. I spin mine with a drill to make it easy. With plug out of course & might be good to remove the blank off in the spare plug hole if fitted just to let the comp out easy to spin. That's with a reverse drill & good if you can slow it down to something like kick start speed too to observe. But spin it faster if you want initially then if it sparks you can slow it down from there.

I cannot try coil on another bike as that bike is a major restoration project
Tried both coils no spark with the plug boot off