What are the holes under the forend on my chassis for? And what size are they?

The drilled and tapped holes on the bottom of the LSS (Gen 1), LSS-XL (Gen 1), HS3, and TAC 21 Gen 1 are for mounting accessories, such as our Picatinny rails with QD sling cup, and the 6" bolted ARCA/RRS adapter. The holes are tapped for 10-24 screws. 

Our V3/V4 Skeleton Rifle Stock, early ESS buttstocks, and Skeleton carbine stock have holes for mounting our 2.5" Picatinny Rail With QD Sling Cup, and our Bolt-On Bag Rider (also tapped for 10-24 screws.)

 Our LSS (Gen 1) and LSS-XL (Gen 1) feature six holes (four are drilled and tapped, and two are blind.) These holes are for attaching a night vision hood (discontinued). The threaded holes are 8-36.