Yeah, "femma" is literally "fiver" in Swedish. (Or "fifth"; "Jag kom femma i loppet" means "I placed fifth in the race".) And "seiska" is the colloquial pronunciation of seven in general, not just that publication.

So "seiskafemma" is just the digita concatenated; "seven-five" in stead of "seventy-five". Doesn't feel all that weird to me. For one thing, other languages do that too. And for another, it's all the "-kymmenen" bits that are too long to pronounce, so leaving them out (when that doesn't introduce confusion, of course) seems eminently sensible. Really not weird at all.