HOPE vacuum chucks.
Direct thread M33 x 3.5,
Large is 6.5-inch dia
Small is 4-inch dia
Made from solid engineering plastic so very strong and of course 100% air-tight.
The face has a slight dome and has a 1mm neoprene stick on the face that gives an excellent seal. Also very easy to change if damaged. Each chuck comes with a spare self-adhesive neoprene sheet.