For the life of me, I cannot remember anyone actually making moulded "P" size endbells -- old age is getting the better of me! I only seem to recall Mura "B" and "C' endbells being turned to size.
In South Africa in the late 1970's / early 1980's, we pretty much followed what the Brits were doing in 1/32nd Open Class. Chassis were brass and wire or stainless-steel and wire "flexi-board's" and "flexi-iso's", and we were using Johnson 111 cans, initially with hand-cut ceramic magnets, and Mura "B' endbells turned to size.
The difference between South African (SAMCA) specifications and ECRA rules, at that time, was that in SA we ran 12mm diameter 'O' ring front wheels and "silhouette" bodies -- with side-dams! (Yes, in the 1/32nd scale!). Everyone used either Betta Ferrari 312P 'silhouette', Betta 'open' Plymouth Superbird or CAT Ferrari 312P bodies -- and the cars looked absolutely dreadful!
Following the trend set by the Brits, in early 1980 spring-steel chassis became the norm, initially just spring-steel centre-sections with pianowire main rails and brass sidepans, but eventually all spring-steel chassis. They were cut by hand, using a Dremel! Then along came the bonded polymer cobalt magnet, moving the development of the 111/13UO motors to an even higher level.
In late 1981 or early 1982, Dave Harvey of 1-O-1 products marketed the InPHinity can in the UK. This was the same size as the Johnson 111 / Mabuchi 13UO can and of excellent quality, in fact far superior to the 'C' cans made today. The can was developed by Pete Hore (hence the PH) and Dave Harvey, and it used a turned down Mura endbell and modified Mura brush-gear, also sold by 1-O-1, along with moulded bonded polymer cobalt magnets. Chas Keeling of SCD introduced the Nexus can at around about the same time (I think) but it was not as successful as the InPHinity.
These were dark years for slot racing in the USA and I don't know what was happening in your part of the world, but I suspect that ProSlot were marketing the InPHinity and some of them were being used in 1/24th scale Group 7 racing. PDL would know.
Here is a picture of Ian Jensen's 1982 British National Championship winning saloon car:-
The motor is a Johnson 111 can with polymer bonded cobalt magnets and a .500" long 26 turns of 26 armature.
Pete Hore's 1982 British National Championship winning sports car, with an InPHinity motor:-
Pictures courtesy of Chris Frost.
As a matter of interest, Team Slot has magnets that they claim are 1,200 Gauss (part ref. TS52038) for their 111/13UO size TS3/TS4/TS5 motors, which are basically "modern" 13UO's....