I have no idea what rails Martin and Tom used. But I am sure about one thing - it is not the rail difference that helped them making those records!
Even if there were some difference in the hydrodynamic behavior, I am persuaded it would not make any significant effect on the total length. Perhaps a few cm, but that's all.
I have C4 Flaps too, but I continuously lost most of the rails - they are too thin and peal off when the fins touch each other in water or in the bag. Presently my fins have only last few pieces of the rails on some parts of the blade edges. However, despite the rail loss I am continuously improving my PB, and definitely do not feel any remarkable difference when training. Well, it does not mean I could not make slightly better improvements with complete rails, but think it is really the last factor that matters.
The rails on my blades are 1.5cm high on one side, hence 3cm in total - and I assume it is the case at most other older C4 fins too. Currently C4 started to produce new rails with variable profile, so the thickness varies over the length of the blade.
Now I do not know if Tom used the same fins at his records, but if you look at some videos, the rails on his Flaps seem to be the same as the ones I have too (2x1.5cm), but, of course, I may be wrong and they may be higher. Look for yourself - check for example the interview with Tom on the video number 04 in his
video archive - he is pulling the Flaps from his bag and showing them to the camera at around 12 minutes into the video clip.
On the other hand, other fin manufacturers use quite different type of rails. So for example Stephan Mifsud, who did some record dynamic apnea too, uses (AFAIK) Breier fins. Those have rails with much less height (about 1 cm) and much thicker - they are in fact triangular.
