[bobH]

There's the 10-spline vs 6-spline pinion deal, which I didn't notice as being mentioned. I opened what I think was a 37 recently, which had the 37-up spring hangers, but had the hollow-tube d/s (which many people associate with 35-36), and it had a 10-spline pinion (which is generally considered for earlier). All I'm suggesting is, hard to tell what you really have, without looking inside. And, even then, you probably won't be able to pin down an exact year. And, after you learn all of this, then, for what purpose?
Go with the basics... 35-36 has spring hangers on the radius rods. 37-40 has spring hangers forged on axle housing ends. No zirk fitting on TT means 35 up to somewhere around 37-38. A zirk fitting on the TT means somewhere around 37-38, up to end of that style (40). Some might say 41, but 41 introduces another can of worms (unique spring hangers). Clear as mud? JMHO