From the 1981 Gun Digest book of values.................
"Model 12C is the same as standard model 12A, except for pistol-grip stock, octagonal barrel. Used value $165-$185."
I have a standard model 12 showing "Pedersen Patent" from 1909-1910.
I'm not sure what you mean by "not arched at the grip". Are you saying that your stock doesn't have the pistol grip? If not, you may not have the original stock.
Here's a link to a website you might find useful, if you haven't already seen it..........
Hope this helps a bit, as it appears you haven't got much help so far.
ps at a big gun show in Phoenix recently I saw two Model 12's on a vendor table. Both were about 75-80% guns, one a Model A, the other a Model C. The A was priced at $995, the C at $1095. My wife found the one I just got for $50, also about 75-80%