300.
I would guess that it was DZ-3 Lacquer Primer?
Good High build primer in its day.( and still is)

I used to use it many years ago , and was a very good primer.
I would use a good PPG grade Thinner ( I think if memory serves me would be the DTL Thinners )as the cheap stuff might not have the chemicals in it to help with longevity of the primer.
A Gallon of good grade thinner from PPG shouldnt be more then 20 bucks i would think.

Buy a gallon of cheap stuff for clean up , and use the good thinner in your primer

Spray