Squarely seated gas checks are a must for accurate loads. This ingenious, patented device allows consistent, square seating of gas checks. Works on all Lyman #450, 4500 Sizer/Lubricators and RCBS Lube-A-Matic. A squarely seated gas check is a must for accurate loads.To this end Lyman has produced an ingenious add-on device for their Lyman #450, 4500 Sizer/Lubricators and RCBS Lube-A-Matic presses. It allows for consistent, square seating of gas checks, the secret to accurate cast bullet shooting.