Well, I am not sure, but I just put a 12 up to a ruler and the barrel looks to be closer to 9mm, the cap around 10mm, just eyeballing it. Now, a 12, 13, and 14 are all essentially the same size pen with a #2, #3, or #4 nib, so I am guessing that if this pen actually is 11mm, then it is at least a 15, possibly a 16 (depending on whether the seller measured the body or the cap). That may be part of the calculation that made it go so high. When I first looked at it I thought it looked a little chunky for a 12, which is partly what made me ask about the model/nib size.
I assume you are familier with these, but if not you may be a little surprised at how thin they are. 12s are really thin - almost exactly the size of a disposable bic.
I am not sure whether it was shill bidding or just a common thought. The underbidder looks like they collect a lot of nice Waterman pens, Waterman ads and ephemera, Vintage Waterman ink bottles and Carter's Ink and ephemera. Also, I was wrong about you winning by pennies - that is only in the low-end auctions I usually bid on

. At the level of your bid, the bid incriment is higher, so you actually won by $1.79. It looks like you both bid $150 plus a little bit extra to beat out the other bidders bidding $150 - only your "little bit extra" was more.
It sounds like a really great pen. We expect a full report when you get it.
John