HELP to get the origin of a IWC with PROBUS SCAFUSIA thks

Hello collector's
i just got from an old lady an oversized IWC.
I need some help because i don't know wich caliber it is? wich year ?
the watch is an IWC vintage huge 40 mm without lugs in solid gold 18 ct with the sign " PROBUS SCAFUSIA" on the movement.
the serial of the movement is 859761
the cover case is not marked with an IWC sign but 2 hallmarks on is FGY or GY and the other is a head of a fish?
the 2 numbers above is 34446 and under is 36
date of the watch? origin? caliber ? should i refinished the dial ?
thks for helping
regards
pascal

sorry after checking the hallmarks It's a head followed by 4
then the other one is FGX ( the F can't be readed easily)

Dear Pascal,

It's a Calibre 74 movement from 1927, but also you have here what we consider a "marriage" --that is, it's a pocket watch that someone later changed, outside the factory, to a wristwatch.

Also, please see the separate e-mail that I sent you.

This thread contains more AD info to a web shop than intersting stuff,
I thought this forum is non commercial....

Sorry

Ralph