CONCLUSION: Booster was starting to fail/maulfunction.
Replaced booster due to "fluttering" and a slowly escalating vacuum leak The "going solid" was not present in the reman booster.
Further the output pushrod of the old booster could be pushed sideways to about a 45* angle - this was NOT possible with the reman one.
The reman booster behaves like a typical booster and does not have any idiosyncrasies albeit that the small diameter results in a "smaller" "smoosh" feeling when applying pedal.
Oh, and there is more stopping power too - nothing dramatic but definitely noticeable.