I've seen 98 Lg ring Mauser bolt with different style vent hole on the underside. Which model is the round hole from? Are the elongated holes are the most prevalent?

Yep; they are elongated until late 1944; then went to round holes by some makers because it is easier to drill a hole than mill a slot. I have al versionsl that were made in 44 and 45 so if you have a specific question about a maker, let me know.