P-51 armament question

While looking at AM P-51B I stumbled across spent cartridge chutes. Pictures I have access to (D&S, In Action, Walk Around) show a bit different arrangement of the openings than what's in the kit - one large and two small ones. I initially assumed that the large one is for cartridges and the small ones for belt links, but that doesn't fit all that well; these small holes seem to be staggered differently then the guns. Could someone enlighten me as to the purpose of these three openings? Also: were they the same on the British version (Mustang III)?

While going through books I've also noticed that P-51D has three larger openings per wing, but only two small ones. Does someone know what these small ones were for? Or why were there only two of them?

TIA, Chris

I can't swear to you that what I am about to say is true because I do not have all my references at hand but it appears to fit. When the British acquired the Mustang I, at least one version was armed with four .50 cal machine guns and four .303 cal machine guns. The .303 guns were mounted in the wings and the .50's were split with a pair on the underside of the Allison-engined nose and a second pair between the wing mounted .303's. It is possible that these holes are properly aligned and sized for the different shell and gun sizes and spacing. It is often true that, as more advanced models of an aircraft come out, some old tooling remains as it is deemed not worth the cost to rework the production jigs. I remember hearing that when Lockheed redesigned the C-130 for the new J model, they finally removed some 2000 lbs of wire that no longer went anywhere but was still being installed because it had never been removed from the assembly drawings. Perhaps North American saw a future in retaining a small caliber gun capability for the Mustang or they decided to simply leave the holes as more cost effective.

Pat

The price of peace is eternal vigilance.