Fender 78 P Bass slotted saddle screws

If you really want a precise fit you might have to go with something like this: Guitar Tech Screwdriver Set | stewmac.com and even at that, I couldn’t guarantee the bit you want is in there. Personally, I would find the closest flathead that’s slightly bigger in my toolbox or at Home Depot then take the bench grinder to it until it was where I wanted it then go back to HD and get stainless height adjusting screws with Allen heads instead of slot heads.

If you have the bridge off or can keep the body from getting soaked with oil, running some Kroil or 3in1 oil down around the threads and letting it soak in a while before you try and remove them should help a lot with corroded screws.

 

If you have the screw in front of you you can measure it or make a rubbing of it with a pencil. Then you just go to the hardware store and choose the correct size.

If the screw is in your possession, you are the best person to answer this question.

 

I've always used these for small precision work. Sadly I lost my smallest one, https://www.walmart.com/ip/STANLEY-66-039-6-Piece-Jeweler-s-Precision-Screwdriver-Set/19901242

 

That's what I use as well, but I already had a set because I wear glasses with tiny screws holding them together. It's a must have set of tools in my case.