From what I've read, Marauder were a company of their own and GW was caster and distriputer of their minis, until later on their ranges were merged with Citadel and Marauder Miniatures ceased to exist. Could be misremembering this or have read something that wasn't quite true, though.
In any case, I thought it might have made more sense (from customers' perspective, propably not from GW's!) to have Marauder slayers sold in separate regiment and later Citadel ones on their own.
No idea about that photo/scan except that it's not great.