[70LonghornCST]

I went to the website and found the link above. It appears that what they are claiming is this:

"HEI and other electronic distributors create a different signal or pulse then do original points style distributors. For this reason, your OEM tach that was circuited to read the latter, does not recognize and cannot read the signal it’s receiving from the newer style distributor. The fix for this situation is to purchase a tach filter or interface device. They are available through several aftermarket manufacturers. They can be found on most GM cars in junkyards that have factory tachs in them. The factory style filter looks like a condenser with a wire and connector on each end, one of which plugs onto the HEI distributor, and the other onto the tach lead wire. Hooking this filter device in series between your distributor and tach should allow your factory tach to work in harmony with your new electronic distributor."

So, in retrospect, it appears that what they are claiming is that it is the HEI, not the wire itself. So, Longhorn Man is certainly correct there. I guess the only way to find out for sure is to hook it up and see. I'll be back in a day or two to let you know the results.