Identify IWC Movement

This manual wind 'pin set' sub seconds at 6 was purchased on ebay staing a dimensional measurement of 26 mm across. My watchmaker measured it at 27 mm across and to the best of his judgement he determined that it was a Caliber 64 by studying a vintage watch illustration book. We are now bidding on ebay for a movement that the seller is positive measures 28 mm. Sending him an enlarged photocopy of the movement he said that, "it looks EXACTLY like the movement" he is selling.

My question is this: Did IWC make several different movements with only a very minor difference in size..Or are we just having a tough time getting a precise measurement of what must be the same movement?

If the forum could answer speedily it would be graciously appreciated as the auction closes this very day. We are writing this request to the forum at the seller's suggestion hoping to avoid a mistake.
Kindest regards,
Ben

Pardon me for neglecting to add vital information for my request for help.
The worn out movement in the IWC we are trying to restore has the serial number:
179764
The replacement movement we purchased recently that my watchmaker said "looks like the right one" is serial number:
737497
***The movement we are considering buying [not running broken balance] is serial number:
641911
The Auction closes in 11 hours. Thanks for your kindness!!!
Sincerely,
Ben

A MISTAKE was made. What we actually have here to restore is an IWC Caliber 63. In error we bought several Caliber 64 movements that were of no use. They are amazingly alike but very few components are interchangeable. PLEASE IWC collectors help me find some IWC parts for an Caliber 63. stroudart@yahoo.com
With sincere thanks,
Ben Stroud

Hmmm....according to my information most of the parts are interchangeable. They are "companion" movements, with one being a Lepine configuration (with winding stem at 12) and the other being a Savonette configuration, having the stem at 3 o'clock. Please see the following parts list from the 1917 parts catalog: