I have one of these, an older model made by Dahlstrom which apparently
was bought by Cedarburg. Mine has the groove also and there is what
appears to be a springy steel wire in it, but since the groove is hex
shaped, the wire only shows at the corners of the hex. There are also
flat springy "fingers" at the upper end of the recess that probably
help retain the tap holders. They do a good job too, I had to put a
short coil spring in the recess to make the tap holders easier to
remove if the little setscrew that holds the tap comes loose and the
holder gets trapped in there flush with the end of the opening. Looks
like even with shipping you got a good deal on yours, I paid $10 for
mine at a local junkyard but a set of tap holders cost over $120 from
Cedarburg.

Mike