When I first saw this piece I thought it was an M-2636 OB. It was a darker glaze than mine. When I picked it up to see if it had a letter incuse on the underside of the top shell, I noticed the lack of a flared collar. Then I looked real close at the taper of the bottom shell and realized it was indeed an M-2643. I had never seen one offered before and the price was right so I made quick work of getting it before one of my fellow Mudders caught on. The M-2643 is on the right. My M-2636 is on the left. Tough to tell the difference without them side by side where it is obvious. I was in the right place at the right time as it had hit the table only minutes earlier.