Saginaw 3-spd Gear Ratio Identification

[jcwren]

I'm having Jim Cook Performance Transmissions & Rear End source me a rebuilt 3-spd Saginaw. I haven't pulled the trans from my truck yet, but from what I read, the inline 6 should have a 2.85:1 first gear, which means there should be no grooves on the spline shaft.

The picture below is from a Saginaw 3-spd off an 1970 inline 6 that I bought about 6 months ago and that I'm using as a core. FWIW, the engine block casting numbers on this engine and the engine in my truck are the same casting numbers, although I can't be sure that either transmission hasn't been swapped at some point.

This one has GM28 / 3925647 / L280 cast on the passenger's side, and the flat machined area has no markings. I'm not sure about mine right now (it's up at the paint shop getting some scratches fixed). My notes says GM51 / NF3899838 and S0R03 (Saginaw, October 3rd 1970). I can't find an 'S' code anywhere on this one.

The picture, to me, looks like it has 3 grooves, making it a 3.50:1 ratio. Am I reading these grooves right? And would they have paired a 3.50:1 with an inline 6?

Any ideas as to what I'm looking at? I have some other pictures if those would help.