These were advertised alot during early 70's.  They are plastic or wooden
"screens" (grates) in the shape of a regular frame, say 1/8 - 1/4" thick.
The mesh is such that the bees can pass through them.  What you do is
reduce the number of frames to 9 (or 8?) and hang one of these dummy
grate-frames in between each pair of combs.  The idea was to increase the
space between comb surfaces without letting the bees construct anything in
that space.  There were big claims that this was the answer to all your
swarming worries, by giving the bees more room to cluster & work between
combs.  In practice they didn't work consistently and the idea was
abandoned from what I remember.  Gleanings ran an article or two several
years ago that basically said "they don't work - don't bother."  BTW they
were marketed as "No-Swarm Cluster Frames,"  and they were pretty expensive
for what they were.