If you encounter a fuel leak at the float chamber mounting banjo bolt, the first suspect is deteriorated rubber grommets. A few years ago Moss Motors began making these grommets with Viton (which is much more fuel resistant than most other elastomers), and these hold their integrity a LOT longer.

If installing new grommets does not stop the leak, check the steel washer on the bottom of the stack. See illustrations below. For the 1500 model this is part number AUC1137. For very early 1600's the part number is AUC2130. I have no idea why the part number changed, but this should be a spherical washer (cup washer), and it MUST be installed with the concave side up. If someone installed it upside down or used a flat washer, it will leak. There is shoulder on the bolt (early) or stud with nut (later). Pull it up until you contact the shoulder (tight). The float chamber should wiggle just a little with a firm push, otherwise quite firm and very secure.

Part numbers on the left come from the MGA 1500 SPL.
Part numbers on the right come from the MGA 1600 SPL.

Engine change point is not recorded, but apparently sometime during early 1600 production. I don't know why part numbers for the banjo bolt, old style cup washer and rubber grommet changed between 1500 and 1600 books. Float chambers and throttle bodies are the same with same female thread for the fuel port, so these banjo parts should be interchangeable as sets.

The bolt and stud will have the same top end thread, maybe slightly longer on the stud to accommodate the AUC1384 brass seal washer. The bolt and stud will have the same barrel diameter to accept the same rubber grommets. The AUC1389 washer takes up a little space on the barrel of the stud; the AUC1147 (or AUC 2130) cup washer takes up similar space on the barrel of the bolt. As such the barrels should be about the same length.

AUC1389 washer (used with the stud) should probably be flat, as it takes the place of the flat face on the throttle body to back up the top grommet. This will have the same ID and OD as the earlier cup washer.

AUC1388 washer will have smaller ID than the cup washer, because it fits over the thread and bears against the shoulder on the stud. I would have expected this to be a cup washer similar to the earlier one, but it looks flat in the illustration. If it is supposed to be flat, then perhaps the barrel of the stud may be slightly shorter than on the bolt.

As a practical matter, when you need to replace the rubber grommets it may be easier to R&R the nut leaving the stud in place, although I have never had any problem to R&R the banjo bolt. The real difference is that multiple R&R of the bolt may eventually wear the threads or shoulder seating area on the pot metal throttle body.

As a casual observation, use of the stud will lower the float chamber assembly by thickness of the brass seal washer plus hex shoulder on the stud plus thickness of the top steel washer. I don't have those parts in hand to say what that distance will be, but this will effectively lower the fuel level relative to the main jet. This maybe some of the reason why the 1600 and 1622 engines use different fuel metering needle than the 1500.