We can wonder at the problems of the Japanese Ordnance Supply sergeant, but was it that different from others? At one time or another in WWII, the German Military and Paramilitary establishment used:
Pistol Ammunition: 6.35mm, 7.65mm Brng, 7.65mm French Long, 9mm Kurz, 7.63 Mauser (SS had Schnellfeuer Mauser 912s) 9mm Para, 9mm Steyr, 9mm Largo (Astras from Spain), 9 x 25mm Mauser, and yes, in Norway, even .45 A.C.P. I have probably missed some. Saw a picture of a German soldier up on a railway right-of-way once, talking to some big shot outside of Warsaw and he clearly had the Austrian Holster of an 8mm Roth-Steyr pistol. Goering was captured wearing a Smith and Wesson .38 Special Military & Police Model Revolver, now in the West Point Museum I am told, that he purchased new from S&W’s German reps in the 1930s.
Rifles and MGS: 7.9 x 33mm, 7.9x57mm, 6.5 Dutch, 6.5 Carcano, 8x56R Austrian, 9.3 x 72R and 20 Ga (Luftwaffe survival drillings), probably 6.5 x 55 due to their involvement in Norway and possibly even 8mm Danish Krag, qnd who knows what else. They were lucky that so many of the European countries like Poland, Czechoslovakia and Yugoslavia had gone to the Mauser rifle in 7.9, as it eased their problem of ammunition supply for much of their captured weaponry, which they had to use.
We have not even considered the Volksturm here, who used just about everything and anything.
America had .30 Carbine, .30-06, .50 MG, 12 Gauge Shtogun (widely used in the Pacific by the USMC), .32 Auto, .380 Auto (both used by General grade officers and OSS operatives), 9mm Para (OSS again, and perhaps others), .45 A.C.P., .38 Special, .38 S&W, and again I have probably missed some. One could go on and on but the plethora of weapons and different calibers wasn’t especially unique to the Japanese. They probably led the way for different calibers among first-line, regular troops, but not even sure of that.
The Chinese in WWII and Korea probably had the Japanese beat. What a hodge podge of equipment. The NVA and VC the same, although I guess towards the end of the VN War, the NVA was getting more and more standardized on Russian caliber weapons. Even the US between Army, USMC, Navy Seabees, Air Force Personnel, advisors, etc. used about as many calibers in VN as they did in WWII.