The traces on many of the SCELBI PCBs travel very close to the edge of the board and the card guides. Since the boards do not have a solder mask, it is possible that one of these traces could short out with the metal card guides. Though the card guides are not electrically connected to anywhere else, this potential for accidental shorts should be avoided. After inserting the cards, I carefully checked for shorts. The only card that has a trace actually touching a card guide, is the input card. To prevent accidental shorts I insulated this connection with masking tape, as shown here.

Input Card Potential Short

I also checked to make sure that I hadn’t made a mistake in my layout, but the original cards also appear to have this potential issue. See this crop of the image of an original input card from http://www.olson-ndt.com/Scelbi/

Input Card Right Edge