This looks like a set of vintage chrome 2+2 M4's with the screw hole on the bottom.
 
http://cgi.ebay.com/Vintage-Schaller-chrome-bass-tuning-keys-machine-heads_W0QQitemZ230348151620QQcmdZViewItemQQptZGuitar_Accessories?hash=item35a1d1db44&_trksid=p3286.c0.m14&_trkparms=65%3A10%7C66%3A2%7C39%3A1%7C240%3A1318%7C301%3A1%7C293%3A2%7C294%3A50

Note this:
 
The set includes the four threaded bushings, but no mounting screws.  The bottom of the threaded bushings have been ground about 1/16 inch shorter to facilitate mounting to my custom-made bass which had a scroll headstock similar to a violin.  The tuning keys will now adjust to fit headstocks from approximately ? to ? inch in thickness.  This modification is completely invisible and does not affect their function.