I don’t know much about this, see photos.
Snap-On 18mm Compression Test Gauge Hose MT-26-11B
Performance Tool Compression Test Kit W80579; Flex Drive, 14/18mm 0-300psi
Snap-on Tools USA Compression Test Gauge Hoses Lot 14mm & 18mm MT26J200+
4PCS W80577 Spark Plug Adapter Set, 10/12/16/18mm Cylinder Compression Test Tool
Snap On MT26 - 11B - 18mm Compression Tester Gauge Hose
