Actually I believe their are 3. One for the Electrohome G07 which will work easily with any monitor and 2 Well Gardner's, There are two basic revisions of the WG NTSC, 85X0172 (P371) and 85X0311 (P600). The difference between the two is the input voltage. P371=+15 VDC and P600=+12 VDC.
I have a P600 which is used on a WG 25"K7000 that was used in a Dragons Lair 2 which has a plug on the chasses to provide power and the blanking signal needed to make the NTSC card work.
The Electrohome NTSC works on +16VDC which it gets from the G07 monitor but you can run it off a separate voltage source.
You can find out more on the WG NTSC boards at this link at MikesArcade.com
https://www.mikesarcade.com/cgi-bin/spies.cgi?action=url&type=info&page=wg-ntsc.txt