Since this question comes up periodically, mounting screws for the SU Fuel pump are part number AJD6155Z -- Screw, pump to bracket -- Hex Head, 1/4-26-BSF X 5/8 (zinc). That is for MGA 1500, 1600, and 1600-MK-II cars (more below)

BSF (British Standard Fine) is the fine thread series of the BSW (British Standard Whitworth) 55 degree threads. All of the fasteners and fittings in the SU pump are BA, BSF or BSP. It is a throwback to pre-war design, before the Allied countries adopted Unified thread standards for the war machinery. They continued making the same fuel pumps after the war, so the threads did not change to Unified standard. The Auster windscreen frame and many of the electrical parts are also like that.

The MGA Twin Cam and all of the "Deluxe" cars (Twin Cam chassis with pushrod engine in 1600 and 1600-MK-II body styles) use a larger fuel pump. That pump has 5/16-BSF female threads in the pump body (same reason as above). The Service Parts List calls for mounting studs with hex nuts rather than hex head bolts.
AHH5805 Stud - pump to bracket
FNZ105 Nut for stud -- [5/16" UNF]
LWZ205 Washer for nut (spring)

That makes this a very special stud with 5/16-BSF thread on bottom and 5/16-UNF thread on top. Don't lose them.

On 3/3/2019, "mackmotorsport@aol.com" [mgatwincam group] wrote:
"I'm pretty sure it is BSF by UNF. Worked on a few twin cams".

Unfortunately AHH5805 part number does not appear anywhere on the internet, other than in the Service Parts List. So until someone might come up with a New Old Stock part still in original package with part number on it, this may remain a mystery. If anyone has a very original Twin Cam or "Deluxe" car with original fasteners still in tact, do tell. Assuming there are studs (not hex head bolts) securing the fuel pump to its mounting bracket, I would like to know if the nuts are UNF or BSF threads.

On 10/23/2019, Jonathan Tucker wrote:
"I am currently resurrecting my TC coupe. I have the original fuel pump and I can say quite categorically that the studs are 5/16 BSF into pump body and 5/16 UNF on the nut side. My pump and car go back to 1970 in my ownership and the car has been off the road for most of that time, and I know the pump has never been changed in my ownership".