Does the wood stock and handguard numbers match the serial number on the receiver? If it doesn't i wouldn't trust the stock disc anyway. The only thing i would worry about is can she shoot accurately. I see alot of these were the serial numbers match the bolts only and everything else is mismatched. This seems to be the norm with the swede's from what i have seen lately. The $300 price isn't too bad either, these have been going up in price slowly too. I paid $225 for a 1907 CG M96 a while back and a local gun shop had a M96 for $400 lately too. For some reason all the surplus guns are priced higher lately too from what i have seen while browsing.
I would tell him there's no stock disc so there's no way of knowing the condition of the bore. Plus there's numbers that don't match also. Offer him $225 and show him cash first. Haggle and go to $250 next if he won't take $225. Its up to you wether to go to a final offer of $275. Remember to show the cash and don't show any extra cash. I would put $225 in one pocket and $25 dollars in two other pockets. Never show more cash than you need to show.
You can buy a replacement stock disc and screw.
Maybe someone will know the history about why these were disassembled and seperated and then reassembled in a non serial number matching manner. The receiver, the trigger housing and the bolt numbers match most of the time. I have one that has no number on the bolt but that was probably done when the bolt was replaced for some reason during the repair or rearsenal.