I've had a couple of these 787's. The first obviously has the wrong crown on it but should be a simple replacement. The first 787 also has the double stamped back cover. It's hard to tell for sure but my thoughts on these is they were left over cases, or maybe just the back covers, from other models that were reutilized by Breitling. No sense in letting them go to waste, right? You can see on the first example how the numbers around the '789' are lighter stamped than the 789 indicating they weren't done at the same time. The 787 was then stamped above the serial number. Totally correct but just another Breitling anomaly. Here's what the 789 case looked like originally before getting changed into a 787.
A really nice example serviced with no issues I would expect to be in the $1300-$1700 range. All the three register watches from this period are going up in value quickly. Always go for the watch with the best dial because it's the hardest part to replace.