While looking at the search results for "a 20/20 vision" in a question on ELL asking whether there should be an article before 20/20 vision, I noticed that many of the results that were the correct context and from sources that were not obviously written by non-native speakers seemed to be grouped between the 1920s and 1960s. For example,
In that state you must have a 20 / 20 vision in one eye , or a 20 / 30 vision in the better eye and at least 20 / 40 vision in the poorer eye (Source from 1935)
Don't let a 20/20 vision lull you into a false sense of eye security. It's a common misconception, say University of Chicago eye specialists, that a 20/20 finding means normal vision. (Source from 1955)
I attempted to use Google's NGram viewer to get a better look, but I struggled with getting it to recognize the syntax to handle the "/" properly and couldn't get it to return meaningful results showing the contexts where the phrase was detected. The examples I could find often freely mixed using the indefinite article and the zero article in the same sentence, and looking at the image of the text, it wasn't an OCR error.
With a 20/20 vision in each eye, the stop sign as well as the word "STOP" could be seen distinctly at 255 ft., both eyes open. With 20/30 vision in each eye it was visible at 200 ft. With 20/70 vision it was visible at 100ft...
Prior to looking at the examples, I thought that an article shouldn't be used vision measurements like "20/x vision". Now, I'm not so sure.
Is it acceptable to use either the indefinite article or zero article with a vision measurement in modern AmE, for example, "The pilot had a 20/20 vision in both eyes." If not, did it used to be acceptable and the usage evolved away from using an article? Is there a difference in BrE?
Because of my difficulties searching the corpora available to me, I feel like I may be interpreting a few spurious results as more important than they are, but I'd like to know one way or the other.