Rob,
The NEL.191.52 JAGUAR 3.5 LITRE.pdf as attached by Roger Mc - (its actually a .pdf of my original of NEL.191.52 than I scanned for electronic circulation) is in fact an answer to your question, noting that NEL.191.52 is undated, but other clues date it to be about 1952. (I have a complete set of all these NEL.191 publications originals, all the Jaguar ones which I have scanned, plus all the other marques not scanned - if anyone wants any others - 1.5, 2.5 litre Mark IV, and 2.5, 3.5 litre Mark V)
The trouble is being published in 1952, it shows the spare parts detail/status as of 1952, and unfortunately my holdings of eralier SU spares listing pre these 1952 NEL.191 series is patchy, and indeed I don’t have anything for SS Jaguar (if indeed anything specific exists).
But note, your particular ‘banjo bolts’, are what SU called HOLDING UP BOLTS - are illustrated as item 66 and item 66A. If you look at the specification tables, item 66 is Part No W1541/3 Holding Up Bolt-front carburetter, and item 66A is Part No 3209 Holding Up Bolt-rear carburetter.
If you look at the note below the Specifications Table, your question is explained… "The original type of Holding Up Bolt (Part No.1541) has a large head .92in across flats, and can only be used with a cork sealing washer (Part No.1542) which fits into a recess in the head. Holding Up bolt (part no. 1541/3) with head .71in across flats is used in conjunction with a single washer (Part no 4642) or the later assembly of three special washers… (note the W prefix to 1541/3 denoted being Walterized - a proprietary form of chemical black-oxidising, which may or may not have been also used pre-1952))
So my reading of all of this is your two holding up bolts with recess machined in head was intended for using just the single cork washer each - the recess contains the cork from speading when tightened, allowing the cork surfcae to seal, and no other washers whether fibre, copper or aluminium. The fibre/copper/aluminium washers were only used with holding up bolts that had no recess/flat surfcae - as used in all later SUs. Clearly SU had an ongoing sealing problem with these holding up bolts, and the Spares Catalogues do reveal three different arrangements…
And a PS. One of the major problems with Burlen’s replacement parts - they only reproduce the latest SU parts and not superseded parts, so you cannot buy new Holding Up bolts with the recessed head - but only later flat head reproductions… I had this problem when restoring a pair of original C-type H8 carburetters, one holding up bolt (second type) was sheared off, the other intact but twisted. Couldn’t buy correct replacements so got a local machinist to make up two from scratch, copying exactly my twisted original - without the twist…