Yrs back I built a rifle using an older flat back action and had the same problem. I posted a question and Fred Moreo answered and said that the dimple and groove on the old bolt heads was for alignment but it was found to be not needed so now there are no slotted bolt heads. He said to just grind out the dimple and drill the bolt head to accept the larger dia. firing pin in the bolt. I did and have not had a problems with it.

El Lobo