I have some experience with non-blockable piano actions. I remember
encountering two different types. One type had a coil spring and the
other had a flat steel spring which held tension on the backcheck. As
the wood aged and lost it's strength the spring tension caused the
action to self-destruct. One of the actions destroyed the flange the
other destroyed the backcheck. On the system that destroyed the
backcheck (found in an Aeolian upright) I had Bob Streicher make
replacement wooden parts. I also replaced the coil springs with weaker
springs. It worked well and the piano is still going 20 years later
with no further deterioration.

Paul Manganaro