For those who saw this item:
https://www.ebay.com/itm/183639487495
but didn't know what it went to (Web searches for "5409818" and
"5009817"
didn't turn up anything useful for me), it turns out to be a "Configuration
2" backplane for a PDP-11/05-/10:
http://gunkies.org/wiki/PDP-11/05#Backplane_versions
with slots for one MM11-L memory unit, and 4 SPC slots.
Noel