Well, instead of the tradtional overarm as we know it today, round arm was the main style (infact the legalised one if I remember correctly)- kinda from a lower trajectory and it looks like a sling, though would have been in a pretty slow motion back then. I know that's not a great explanation, but I suppose Malinga of SL is the best current example of it, although the actions adopted 150 years ago would have been very different from that even....
Never heard of a round arm lob?
Anyway, from everything I've read, Grace was referred to as a round arm/slow bowler.