Papa3 wrote:
... Uneventful, that is, until the towpilot came walking over. It
didn't take a genius to figure out that he was not in the best of
moods. I noticed he was carrying something in his hand. The
something was the mangled release mechanism. We had broken it from
the towplane at the mounting bolt; it stayed attached only thanks to
the release cable.

You are lucky you broke the tow hook attachment bolts. This engaged the
tow hook release by pulling on the release cable from the other end.
That is why the hook stayed attached to the towplane, and the rope
stayed attached to the glider (and the tail stayed attached to the
towplane).

P.S. I saw a similar failure on an L-19 tow hook installation due to
metal fatigue. A bolt broke, and the tow hook released during a normal
tow. It happened early in the tow, so no accident. But it showed an
obvious design flaw with that installation. The hook was mounted
directly to the leaf spring with two plates on either side, all
sandwiched together by two 1/4" bolts. The plates were separated by the
spring thickness, and put a lot of shear loading on those tiny bolts,
both of which are critical. I would rather see a 4-bolt installation,
so you get a chance to catch a broken bolt on pre-flight.