Was just asking a suggestion when to know the changing time could be applied, but now that i think of it, i think i could narrow it down to an average period of time, maybe at 1809 - 1810, no need really to be so squared about it. Besides, to be honest i have never reached a year past 1808, campaigns to me turn to be pretty easy to achieve victory.
About the regulation vs reality i think we have to stick with the regulations and slightly side by side to reality, but only if it's clearly stated in or by credited sources. I don't know about the example of the british you gave but i do know and read that austrians couldn't use the shakos because of what you mentioned, supply issues.