X61s slimline battery internals

[vavet]

Hi, I know that it is possible to replace the cells in an old x61 extended battery.
Nowadays there more modern 5000mAh battery cells available which may give about twice the time of the original ones which are 2600Ma mAh. I have opened one of my dead 8cell extended batteries and the cells used there are the standard 18650 ones (with flat heads). Now as I am using also an original slimline 4 cell battery for my x61s, I was wondering what are the cells used there. It is still in good state, so I don't want to open it until it's dead. They are definitely not the 18650s.
Tried searching online seems no info is available. Can anyone tell?