There is a distinct possibility that the wikipedia has promulgated what seems to be a misstatement of a sentence in the highlighted portion of the
well-known old book about the "Cast-Iron Wonder", the "Stove Bolt Six" engine.
There were no slotted head-bolts on the first generation stovebolt engines. However, the sheet-metal covers attached to the engine (valve cover and side covers, and a few other parts) were attached with
1/4"x20 slotted-head bolts, that were/are commonly called stove bolts. They were not head bolts (they were not the bolts that held the head to the block).
Here is a little historical article about the Stove Bolt Six (it also misstates the existence of slotted head-bolts but it is otherwise an interesting write-up).
Tim
