Yes, it's all over the place, and is nearly always attributed to "an MK catalogue", although I haven't seen anyone able to say 'which one'.EDIT: That exact quote also seems to be all over the internet, on many different forums - being the only statement available about double socket loading.... and without naming an actual source
However, it is so specific and detailed that I have to assume that it is real (and probably 'from MK') rather than 'fiction'.
Having said that, I find it a little hard to reconcile "... all MK double sockets can safely withstand a continuous load of 19.5A for an indefinite period." with "...Testing showed that a load of 22.3A was sufficient to cause heat stress that would cause a browning of the faceplates and sufficient heat to cause insulation damage to cable cores.". Furthermore, if it's really true that 22.3A can cause "... browning of the faceplates and sufficient heat to cause insulation damage to cable cores.", then I would start to wonder whether BS1363 is necessarily safe/adequate in only requiring a product to pass a temp-rise test at 20A (for one hour).
Kind Regards, John