Could anyone please explain what this axle is and how it's different from a normal one

Have a read of this

http://www.landyzone.co.uk/lz/f6/difference-between-standard-salisbury-axle-80309.html

In terms of strength its superior to a Rover axle. 4 pin HUUUUGE internal sun and planet gears and a huge spider. Down sides theyre heavy and you loose an inch of ground clearance over later coilsprung axles. there is not a lot in it with older 70's coil sprung axles and series II's as they have less ground clearance than later. The diff pinion is longer again stronger huge bearings. which meansyou need a shorter propshaft. Take more fettling than a rover diff to setup but once done pretty bullet proof

Thanks good to learn more about land rovers all the time