Whilst organizing the mechanical/small engine side of the shop, i came upon a bin of my least used sockets... 3/4 in drive sockets between 1 1/2-3" mostly... I came upon a socket I forgot that I had, it is 3/8 drive with 2 nubs on the outer rim across from each other. I remember seeing this type socket somewhere but I cannot remember what it is used for... Any help? It has no capacity for any kind of hex fastener inside, just smooth and round. Made by MAC tools, it has SC21 stamped into it. A search of MAC tools website gave me nothing.

http://img59.imageshack.us/i/mysterysocket2.jpg/
http://img63.imageshack.us/i/mysterysocket.jpg/

Looks like its a tube socket, that may have been made/modified to undo car aerial mast nuts. They have the mast going through the centre of the nut where it secures to the guard (fender). The nut has two opposing slots on it. I'm in the motor trade, and have used similar sockets. The "21" sounds like it was metric. I have also seen brass cap screw type bolts with a similar head. It was on some sort of old machinery or engine.
Jason

I use one that I made. I think the actual name is "slotted nut socket".
I repair pnuematic nail guns as a side line, and the trigger assembly on a Hitachi NR83 framing gun has that on the trigger cartridge.

Whilst organizing the mechanical/small engine side of the shop, i came upon a bin of my least used sockets... 3/4 in drive sockets between 1 1/2-3" mostly... I came upon a socket I forgot that I had, it is 3/8 drive with 2 nubs on the outer rim across from each other. I remember seeing this type socket somewhere but I cannot remember what it is used for... Any help? It has no capacity for any kind of hex fastener inside, just smooth and round. Made by MAC tools, it has SC21 stamped into it. A search of MAC tools website gave me nothing.

http://img59.imageshack.us/i/mysterysocket2.jpg/
http://img63.imageshack.us/i/mysterysocket.jpg/

Highlander, I have one a little bit larger that was made to use on a free floating axle bearing spring loaded retaining nut (86, 89, 97 ford truck dana axle). The socket that you show looks very similar.

Upon further research, it looks to be a socket for turning spanner nuts, were used all over the place on 60's-80's gm vehicles as well as in modern appliances and a host of other pieces of equipment. Thank you everyone for your knowledge and expertise in helping me track down the purpose of this socket.

i bet if ya keep it you'll never use it. but throw it away and need it the next day...how my luck goes....

Made one like it about 25 yrs ago. To get the pump assy out of a laycock overdrive
Haven't found another use for it yet.
Ken.