We already know that machine shops are making mortars and mortar shells by 1999-2000 time period - obviously they either had to reverse engineer existing designs or use existing blue prints of older designs.
And you have people scattered around the US who restore older equipment who would have various design drawings (either copies or originals) - those could be used to restart production of older equipment (at very low levels of production - i.e. basically hand built)
and you could go back to things like using rivets to make armored vehicles like they were made in WWI and early WWII instead of modern techniques - again at very low rates
I would think the place you would see old designs coming back the quickest would be either cannons as they were around the 1860's and older weapons that a gunsmith could easily make - muskets and the like - and while that means stepping back into the 19th century those weapons would still be very effective in many cases
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