This pressure cap would be called a radiator cap if installed on your car. It does differ from an automotive cap in a couple of important ways.
1) Its pressure rating is 14 or 15 p.s.i. (engine supplier uses both interchangeably)
2) Since marine engines are always short on available space, height is a concern. To address this concern, the riser on the coolant reservoir is shorter than an average radiator. If you attempt to fit an automotive cap the your engine's coolant reservoir, it is likely the cap will hit the coolant overflow tube and prevent it from working.
Applications include these engines and the boats that we are aware used these engines... There may be other combinations as well:
• M-12 (Catalina 25, Capri 26)
• M-2-12 (Catalina 25)
• M-18 (Catalina 27, Catalina 30)
• M-25 (Catalina 30, Catalina 34, Catalina 36)
• M-25XP, M-25XPB (Catalina 28, Catalina 30, Catalina 310, Catalina 34, Catalina 36)
• M-320 (Catalina 27, Catalina 28, Catalina 30)
• M-35 (Catalina 34, Catalina 350, Catalina 36, Catalina 38)
• M-35B (Catalina 34, Catalina 350, Catalina 36)
It does not fit the cooling system of a Perkins diesel.