Need help to identify IWC from 1970's

I inherited an IWC watch from my grandfather.

I'm told that it was originally purchased in the 1970's.

When it was given to me, the back was missing and not in working condition.

From what I'm being told from IWC is that the only option is to find another IWC watch and use that back for this one.

Problem is, I cant find what model it is.

I believe its either a 34 or 35mm, gold watch (see attached photos)

I can take better photos if needed.

The only number i can see on it is 1391129.

Any help would be great!

Amazingly beautiful IWC Cal 89 from 1956. 
There are some specialists who are able to manufacture casebacks, but you will need to search out one who will do it for you. 

The other route is to source a trashed watch but the chances are it will be difficult to find the exact size. 

Have you considered sending the watch to Schaffhausen for service and restoration. Maybe then IWC might be able to source/supply a replacement caseback. 

Thanks Mark for the reply!

Very interesting that its a 1956, sounds like my Grandfather purchased it as a vintage watch as well!

How would i go about determining the actual size?

I have seen other watches that look really similar.

Are the case backs on these watches the same for multiple styles of watch? 

Or are they unique to that watch/style/size etc? 

Hi - just for clarification, Mark was pointing out that the movement in your watch is from 1956, not your watch. Based on your dial - with the block IWC logo- your watch would have most likely been assembled and sold in the late 60s or in the 70s. Without a case back - where the case number is engraved - it’s not possible to know when the case was made but it’s normal for movements to predate cases.

As for finding a case back, you’ll need to find a watchmaker who can determine your options. 
Good luck!

T

Thank You Terry for clarifying. 
The watch face is certainly a decade younger than the 1956 movement. 
My first impression when seeing the dial, was that the movement was Cal 401. Only confirmed as 89 by dating the movement number.