A week ago I read an interesting thread about putting a 6117 GMT movement into a 6309-729A case. This thread also mentioned that some sellers on eBay where selling GMT equipped 6309-7040's and the movement was still a 6309. I had to look more into this, so I searched on eBay and indeed I found photos of 6309 movements with 4 hands! Further research yielded this great article => 6309GMT: a how-to of sorts.
I always wanted a modded 6309-7040, and after this I had to have a 6309GMT powering it
A few days ago I managed to buy such a watch. The 6309 movement has been serviced and modified to have a working GMT hand and hack. New gaskets have been installed, but there is no guarantee on water resistance. And thankfully from the seller's photos the case and case back don't look like they have been over polished.
I plan to modifiy this watch further, so watch this space!