The usage of the term "holotype" here is to identify a specific physical object as the prime example of a general class of similar objects. The similarity to the holotype, is the measure of identification with the general class of that kind of object.
This term is used extensively in science. For example, for each species of marine animal, there is a presenved specimen curated somewhere, typically in a Natural History Museum, kept secure and identified as the holotype of that species, by which any question of identification of some other found animal can ultimately be determined, if other means are not adequate to answer the question. The taxonomy (physical appearance characteristics) of the species was the way to identify a specimen in the past; yet more recently, the newer technology of DNA analysis, performed on the holotype or its "allotypes" (specimens deemed very similar to the original holotype and considered a bit more expendable during analysis) have in recent times found that in some cases specimens looking the same, were actually not the same species.
In the context here, the term "holotype instrument," "paratype instrument" or "allotype instrument" refers to a specific physical instrument in each case.
Similar instruments given the same name, such as "Audio Programmed Zapper," could therfore be compared to the "holotype" instrument, for accuracy as to construction and functionality, in cases of question.
Usually instead of this direct physical comparison, people rely on descriptions of the holotype, so as to determine comparability.
This could be especially useful in the case of instruments built by the individual experimentor or for him/her, from scratch or from a kit. Usually the accuracy is determined by the output signals measured under various test conditions. In the case of manufactured instruments, the manufacturer performes Quality Control inspection of each instrument, as part of the manufacturing process.