Cross recessed (+) flat head screwSmall head(D=7D=9(Coarse Stainless steel Unplated 4 X 20 Pitch 0.7

It has a head shape similar to a flat head machine screw, but with the upper portion of the countersunk head cut away. This reduces the head diameter, making it usable in narrow grooves and other tight spaces where a standard flat head cannot be used. Because it is commonly used in window frame sashes, it is also known as a "sash screw."
Even with a small head, the countersink taper angle is 90°, the same as a standard flat head machine screw.
To achieve a flush finish with the mounting material, it is necessary to pre-apply a countersink machining to the material that matches the head shape. Because the head is small, only a small countersink is required, making it well-suited for thin sheet materials. However, note that the smaller head reduces the contact area with the material, resulting in lower clamping force.
Machine Screw Head Dimensions List　Encyclopedia of Small Head Flat Head Machine Screws　How to Measure Flat Head Screws (Nejikuru Facebook) Flat Head Machine Screw Small Head

• Cross recessed (+) flat head machine screw small head (D=7 D=9 (Coarse
• Nominal diameter (d): M4
• Length (l): 20.0
• Pitch: 0.7
• Cross recess number: 2
• Head diameter (dk): Nominal 7.0, Tolerance 0,-0.5
• Head height (k): Nominal 1.75, Tolerance 0,-0.3
• Head chamfer section (c): Approx. 0.25
• Cross recess (m): Reference 4.1