(-)Old JISRound head machine screw Brass Unplated 3 X 6 Pitch 0.5

The head of a round machine screw is nearly hemispherical in shape, similar to today's truss head machine screw. The flat head machine screw has a low-profile cylindrical head. It was commonly manufactured in the past due to its simple production method.
Machine Screw Head Dimensions List　Encyclopedia of Round & Flat Head Machine Screws

• Slotted (-) old JIS round head machine screw
• Nominal thread diameter M3
• Length under head (ℓ) 6.0
• Pitch 0.6
• Head diameter (dk) Nominal 5.5 Tolerance 0,-0.5
• Head height (k) Nominal 2 Tolerance ±0.15
• Slot width (n) Nominal 0.8 Tolerance +0.15,-0
• Slot depth (t) Nominal 1.2 Tolerance ±0.2