AP Lockheed supplied a 5/8" i.d. master cylinder to work the original steel single disc caliper that has two 1-5/8" o.d. pistons.
On my T160, that combination was underwhelming when using the engine's performance,
more so when it was being used to move two people and sometimes enough luggage for European touring ... I fitted an additional front disc/caliper, the latter also having two pistons the same diameter, and a 0.7" i.d. master cylinder also supplied by Lockheed. The combination has proved excellent.
In relatively recent years, it has become fashionable on internet forums to disparage the Lockheed combinations and fit smaller i.d. master cylinders - the 5/8" i.d. cylinder with twin calipers, cylinders around 12 mm-1/2"-13 mm with a single caliper, to the extent that at least one wholesaler - Wassell 'Girling' (in the UK, Emgo in the US?) - commissions the manufacture of 13 mm. i.d. cylinders the same o.d. and mounting thread as Lockheed cylinders.
I have tried bikes with those non-Lockheed combinations; ime, they are uniformly awful ...
It could be me - I owned a 79 Honda Dream from new, their front brake was infamous; in the UK, annual vehicle safety certification (the "MOT") starts at three years old, my bike's front brake failed that first certification
because the lever could be squeezed to the throttle grip despite an overhaul ... The Honda master cylinder was 14 mm i.d. and ridiculously expensive, I fitted a Lockheed 5/8" i.d. master and a length of Goodridge hose, transformed the brake into something I was very happy with and continued to use for another six years.
While I do not intend to hijack your thread in that direction, as an aside, the new OIF - developed at the same time as the conical hub forks - required new manufacturing processes. Similarly the conical hub aluminium alloy fork sliders and hub/drum - aiui, BSA Guns devised the sliders machining ... Otoh, the conical hub yokes were similar steel or aluminium alloy castings to the yokes for the previous forks, so there would have been little in their manufacturing to redo; just two years later, BSA/Triumph changed the yoke castings again to move the fork legs nearly an inch further apart to accommodate the disc brake.
Also, Umberslade Hall was specifically intended as a BSA Group r&d centre and its location was partly chosen so it was not particularly near either the Triumph works at Meriden or the BSA works at Small Heath (Birmingham) so "we have always done it that way" influence from either works was reduced to a minimum. Sadly however, that also worked against BSA/Triumph, in that most of the development team certainly at Meriden - including talented chief engineer Doug Hele - refused to move to Umberslade Hall; probably one reason why no one checked the Triumph 650 engine could be fitted in the new OIF on the production line, leading to a lengthy delay while cylinder head, rocker boxes and mountings were redesigned, just when Meriden should have been assembling 650 twins to be shipped to the US ready for the 1971 selling season. This alone was one of the major reasons for the BSA Group's subsequent financial collapse.
With hindsight, undoubtedly BSA/Triumph should have fitted front disc brakes certainly to the triples and probably the Bonneville and Lightning twins, the parts adapted for the disc brakes from 73 had been supplied by Lockheed in the early 1960s for tuned Cooper versions of the original Mini car.
However, it was not certain for several years after Honda had produced the CB750 with a front disc that they were really a good idea - stainless steel discs worked well enough when dry but were scary bad in the wet (the latter something of a problem e.g. in the UK
) and, if you remember, e.g. the first versions of bikes like the Suzuki GT750 and GT550 triples had front 4ls drums ...
E.g. 
