re Albion boxes and clutches, they were often used with JAP engines in the 50s ( partly because there was a fashion around that time for using the Royal Enfield swing-arm frame for grass-track bikes, a very light and strong frame by the standards of the day ). Strong but heavy, and largely superseded by the lighter and more robust AMC units in the 60s.
I suspect the slipping issues are related to different styles of track preparation and generally very different machinery. Speedway bikes are extremely light by the standards of US machines, it is common to have a clutch adjusted almost at the point of slipping; the theory is that if your front wheel is JUST off the ground as you leave the line, you are transmitting all the power you can use. Given that even a 50s speedway JAP will only weigh 88kg or so ( modern bikes a re restricted to a minimum weight of 77kg ) and speedway tyres are a trials-type with a lot more grip than a US flat-track tyre ( especially the 22" ones used in the 50s ) then this is necessary.
Track preparation differs, too. US style "blue groove" tracks are not seen, it is usual to dress the track with fresh shale for every meeting and grade the track several times in the course of the meeting. A track curator allowing a US style "blue groove" to develop would be considered hopelessly negligent! Having a clutch which breaks away at a crucial point so that if you find some grip, you have a buffer between you and abruptly looping the loop is both safer and faster.