If you look at this recent thread on here you can see who has such a dividing head and perhaps send each one a PM. They might not look at the forum often enough to see this particular thread. If you get a definitive answer remember to post it on here so that future searches can find it.

I imagine 14 or 16 DP would be about right. You could try a couple of imaginary setups and see if they stretch across the gap without a huge idler. If looking for some lathe changewheels to use the centre hole might be more important as you need to be able to bore out to fit, with or without a sleeve and still have some meat in the centre boss of cast gears.

Well if you've got a gear (presumably you have if you've been trying DP gear gauges on it) then just measure the O.D. in mm and use:-

O.D.= (no. of teeth +2) x MOD

as a check:-

MOD = 25.4/DP

so a comparison with the nearest DP gauges should give you confirmation.