Stainless Steel Plate Lock for Accordions. The set comes with Thumbscrew 5/16" diameter x 1" length.
The lock plate and thumbscrew are sold as a set, but you can also purchase them individually. Options are given above under the title "Buy More Pay Less".
HV Plate Lock with HV Thumb Screw
REMEMBER to choose the correct option and put the correct quantity in your cart when you place your order.