Acquired this the other day. Look carefully at the crown on the non-denomination side and you will see incuse figures beneath - i.e. they were there on the blank before the main design was struck onto the planchet.
Although most books that mention this error (including both KM and Myntboken) describe the figures as "50", it seems they should actually be read as "5Ö", i.e. 5 öre. I'm far from clear how this error arose, but one suggestion is that an "identifier" punch (to distinguish the 5 öre from any other denomination) was wrongly left in the machine when it produced the planchets for the coins, which were then struck in the normal way before anyone noticed. But if that was the case, wouldn't the 5Ö be back to front?