Regarding British extended magazines, these were manufactured in reply to Fritz's magazine extensions and tried out the the Front. There was a 15-round and a 25-round magazine tried for the SMLE, but both had problems with being too delicate, and they really got in the way of prone fire.
It was decided to wothdraw the few extended magazines that had been issued. They are super-rare today.
Not that they were really needed; you can refill the m,agazine on a Lee-Enfield awfully fast if you have decent chargers, faster, in fact, than you can change magazines. This is why it always surprises me when guys just gotta have spare mags for the Lee series..... and when they "borrow" mine and never give them back!