How to tell the actual Year of Commercial Model A's

[Fullraceflathead]

I thought I read somewhere (Can't find it now) that the Commercial Model A's didn't change years with the beginning of Jan. I thought they changed years in May or sometime later than the beginning of each new year.
Is there any way to verify this?
I have a 1928 or so I believe, Roadster pickup. The engine dates it to November 1928, IF it's the Original engine. No the frame numbers never been seen. I'm putting this back into the California DMV System soon (Good luck!, I know).
I was told it was a 1928 by the last owner, he had it since the '60's.
It was originally an abandoned Model A found out in the Desert in the early '60's. The history before this is long gone. It was registered in California up untill the '80's. It was parked until I bought it last year.
Could this still be a 1928 or most likely a 1929? The Pink slip from the '60's is lost and it was listed as a 1929. With the high volume of Model A production in late '28 and early '29 I can't see how this engine sat around for too many weeks before being used. This also assumes it's the original engine.
I'd like to hear some thoughts on this if someone can help.
Thanks,