The change was both chronological and related to use, apparently. I think earlier new series engines all got seats, then some dropped to exhaust only and late passenger car engines got none. Trucks got 16 til the end. I believe Model A's and B's manufactured after some time in the mid-thirties (they were produced for industrial use, diamond blocks) got hard seats, probably at the same time production V8's got them.
All the early flatheads after 193? got 16...I don't have any from that mid-thirties period and I'm not sure when they became standard.
There was a general dropping of expensive Ford features as cost cutting after 1947, as the new accountant-dominated structure took over from the engineers.
Some 59 engines were made for a few years after 1948, I believe, for the parts market, and I have heard of instances of those turning up without seats.
On the late engines, finding seats has become almost random since most blocks by now have been through exchange rebuilding systems and are not in their original chassis.