Evening Ed
This one is easy and no eye strain!!
You are looking at the Oak Park Box Joint & Dovetail Jig.
The spacer rail fence is available in 3 different sizes 1/4", 3/8" and 1/2" and used for accurate cutting of box joints on a table top router surface.
The fence is attached with screws or clamps to the surface over the router plate and then aligned or spaced the appropiate distance from the matching spiral router bit ( ie 1/4" space with 1/4" bit, 3/8" space with a 3/8 bit , and 1/2 " space with a 1/2 " bit )
Once the first cut has been made the workpiece it is then move over the raised rail and will be in the exact position for the next cut, and continue until the end of the workpiece.
The set up for dovetail joints is similar.
Router Workshops has a shop tip with the complete instructions of using the jig at the following url
http://www.routerworkshop.com/boxjoints.html
The spacer fence and rails can be purchased from Oak Park at the following url for CDN customers:
http://ca.oak-park.com/catalogue.htm...ref=cdn12.html
and for USA customers at:
http://us.oak-park.com/catalogue.htm...ref=usa12.html
Hope this is what you were looking for.
Ric