i can't answer why they have different part numbers, but my experience is with the pressure plate that has the three lever arms. That's what came with my TR3A when I bought it brand new in 1958. I remember changing the fingers about 1964 and again in 1992. Also at least once, O used my MIG welder to build back the rounded end and grond it back to where it was usable for another 20,000 miles or so. Then I bought a new full kit from Roadster Factory with the same three fingers. I find it has always engaged very smoothly.
I once drove a TR4A and as I was letting the clutch out, nothing happened - then all of a sudden, it engaged (or should I say - it clamped suddenly) and I prefer mine with the three fingers.
I have heard that the owners who race their TRs prefer the sudden type of clucthing action for a quick start. I have also heard that some replacement clutches for TR4 to TR6 with the multi-star shaped relese levers are from Saab. And these are heavy to cope with the 150 HP and more that these clutches need to have.