I think the quantity of rounds making up a case-manufacturing lot were much higher than 180,000 rounds, generally speaking. Of course, I suppose if a certain contract called for 1,000 rounds, that would consitute a loading lot - just not sure.
The German Group of ECRA has, for years, been working on a complete list of all known German case-manufacture lot numbers. The nice thing about them, is that once you find the highest number, you know how many lots were produced, since it is a safe assumption in most instances that the lots start with one. There is an exception to that with a company that had two factories and used the same letter manufacturer’s code for both factories, but assigned lots 1 thru 49 to one factoy and lots over 49 to another. As you perfectly pointed out, that doesn’t mean that specimens from all still exist, or were even loaded. It is possible that entire case lots were scrapped (although in many instances, bad lots not suitable for the pressures or tactical use of “live” ammunition were loaded as blanks for training purposes). Some of the very best experimental-round headstamps, like the “aux Probe” headstamps, are found more often, or only, as blanks, since once the experiments were over, remaining cases were thrown into the pile to be loaded as blanks.
Regarding the German list, my print-out of the last one sent about a year or so ago fills three, two-inch binders. It shows the lot numbers and the loading, and in most cases even the primers. Of course in the case of many lot numbers, there can be five or six variations of loads with them. Remember, most lot number studies are only talking about the lot number on the headstamp, which along with the manufacturer’s code, does not necessarily relate at all to the loading lot number, the lot number of the primer, bullet and powder, nor even who loaded the ammunition.
Your theory that some lots were completely consumed in combat is pretty sound - at least almost completely consumed. Some always escape, but aren’t necessarily ever found by collectors. Your theory that some lots simply were inaccessible to collectors since WWII because of where the ammunition was shipped is spot on! I had noticed that a huge number of P25-code lot numbers, right in the middle of the series, (and I am taliing about 40 or 50 lots) had not been reported by anyone, and were missing from my collection as well. Some years ago, I think it was Navy Arms imported a huge amount of mixed ammo of caliber 7.9 x 57 from one of the Balkan countries - they wouldn’t report which one but I suspect it was Bulgaria. At any rate it was a country, for sure, where no Western collector had any access at all to ammunition, and not much to the country itself since the close of WWII. You guessed it. Virtually all of the “missing” P25 lot numbers were found in this ammo shipment. It had obviously all gone to whatever country it was found in, either in the hands of German troops, or in my opinion, more likely as aid to that country. Albania and Bulgaria were both German allies in WWII.
Lot numbers are an interesting study, and probably endless.