Bob,
I apologize; I did not pay careful attention to the 1925 usage.
In fact, it does not count, not in the sense of Joe's usage as I understand it (knee-jerk reaction).
The 1925 usage of "knee jerk," as in "gets only an occasional knee jerk" -- note as noun not as adjective -- was in the language, but as "involuntary spasmodic reactions of the limbs or features, esp. resulting from religious excitement." (OED)
This usage in fact dates back to the 1800s. Joe's meaning differs, and not only because his was a modifier rather than a noun. I guess it could be extrapolated into a "knee jerk reaction" but that's not the usage.
So -- wait, you're baiting me, aren't you?
$#%$%$@#,
Mark