spider/diagonal holder question

Hi all--
I have what looks like an old astrosystems spider with a diagonal holder for a 3.1" secondary. My question is, what holds the secondary in place? I've always built my own spiders and used silicone--this is the first commercial holder I've seen, and it has no obvious retention screw or whatnot. The depth at which the hub sits in the diagonal holder is adjustable over a 3/8" range, but it doesn't look like the hub would bear on the back of the secondary unless the secondary were very thick. What is the usual thing here?
Thanks,
George

A big blob of polyester batting, like they use
to stuff pillows. It just fills up the space
and presses the mirror against the front
retaining lip on the shell.