Okay, to solve this mystery....
1) I was able to get out to the garage today and get to this pump (see original pic at the top of this thread). I opened her up and Tom and Matt are correct. This is definitely a cut down 646. The bottom of the door is straight, it does not have a finished edge with a lip, and is rough to the touch. It was obviously cut. The factory must've offered a tall 646 'no ad-glass' version (not pictured in Jack's book).
2) Coincidentally, another one just popped up for sale in Michigan. I was thinking about buying it and restoring them as a matching pair. The gentleman was very nice and sent me pics of his, including a close up of the tag. It reads.... 646.
Here are some pics. The first pic is my pump opened up of the cut down door (it looks like a lip on the door, but it's just a shadow on the ground). The second pic is the one for sale near me that is the same. Notice how the bottom of the door looks like it was easily pushed in, because of no reinforcement / lip. The third pic is the tag on the one for sale. You can clearly see 646 at about one o'clock in the picture.
Thank you all for your input on this matter. It was fun solving this (even though some of you already knew the answer
As for buying the other one near me, I'm still up in the air. I have posted mine for sale. If it doesn't sell, I might buy the other one.
Thanks again Old Gassers,