aluminum blocks with grey dots

by Howard
(Baltimore, Maryland)

I would like to know what are the two matching aluminium blocks are that came with some machinist tools i bought on ebay...the two blocks are about 5 inches long and 1 inch wide with a groove running down the middle of each lengthwise and both blocks have round grey dots about 3 sixteenths in diameter,,the blocks seem to be very slightly magnetic and seem to be a lot heavier then aluminum of that size,,there are no mounting holes. Any help would be appreciated,,thank you,,,Howard