If you've removed the rocker cover to look at the springs just look at the markings on the cylinder head. GX will have no numbers and instead two words 'Japan Nissan'. It will save you having to look at the ports etc.

Sounds like a score, upload a picture.

All A12 have the 13 bolts fro mid 1973

Dual springs are good for 6400 rpm redline instead of the usual 6000 redline.

GX motor has dome-top carbs.

Sounds like a GX motor. Did it come with the cast iron exhaust manifold? If true, you got a deal.

I just bought a motor today. The guy I got it from said it was a JDM Spec motor from Japan...What's that mean ??
I got it home and took off the valve cover and it has dual valve springs and the number 13 on the head bolts,I remember reading something about 13 on the bolts and I know dual valves mean something good ? And it came with the dual SU carbs.
GX Motor?
SSS motor?
Today I will take the manifold off to see what kind of ports it has.