In this construction, off is redundant in meaning.
It is not an acronym.
It is occasionally used in specifications or ordering, after a number, to indicate that the quantity is of single items, not dozens, pairs, hundreds, etc. By doing this, the format n+multiplier+description is always satisfied, even when the multiplier is 1, so the reader knows that it isn't missing.
I believe it is less common in English now than 40-50 years ago, but is still used in other languages e.g. Swedish stycken (abbr. st.). It may have crept into your technical document as a translation, or by an author being more familiar with usage in another language.