Not a native speaker (or an expert on guitar paint jobs) but I think "lacquer" is the umbrella term commonly used for any hard finish based on non-synthetic resins, sometimes using solvents, confusingly acrylic lacquer is based on a synthetic resin but still referred to as "lacquer" and was used on guitars as soon as around 1950, so the only way to avoid doubts and uncertainty is using "nitro" if not "nitrocellulose lacquer", or simply "NC".
If you want to refer the top (clear) coat that is, the stuff below that NC layer could be anything even in ye olde days. For example, by 1960 Fender used some (highly controversial) filler called "Fullerplast" on the wood and if they sprayed e.g. Lake Placid Blue onto that - LPB is acrylic lacquer, which then got some nitro clear coats on top. In other words, even very few "vintage" guitars were true "nitro" from wood to top coat.
By 1968 the finishes were mostly "poly"(urethane) and I'm afraid they even used polyester during the 70s. Similar to "lacquer" people are unfortunately not distinguishing between polyurethane (PUR) and polyester (PE) and call that all just "poly", which doesn't do thin PUR finishes any justice. The guitar finish technology and terminology is complicated and the oversimplifications caused a lot of confusion and misconceptions, so it may be some improvment to simply use "NC", "PUR" and "PE" or the words these abbreviations stand for.