But only the single largest serial number, and the number of samples are being used. Not any of the serial numbers seen besides the largest one.
That means any such noise as you describe will have no effect. As long as a high-serial number tank is ever destroyed or captured, the method will work. It only takes one.
You can get away with only using the count and the maximum because of the assumption of uniformity. Once that assumption goes away, the assumption no longer works.
If earlier tanks are much more likely to be captured, then the number of samples will be exaggerated compared to the maximum number seen, so you'll underestimate production. If earlier tanks are much less likely to be captured, then the number of samples will be smaller compared to the maximum, and you'll overestimate.
I wonder about the potential for error due to bias in the collection process. For example, I would expect older tanks to be more prone to breaking down due to mechanical problems, which might change how likely they are to be captured or destroyed. (I can't tell if that likelihood would go up, because they're more likely to break down in combat, or if it would go down, because they're more likely to be in the shop instead of on the battlefield.)
Evidently it didn't affect the accuracy of this technique too badly, but it makes me wonder.