Broke key guide on 6sn7 how to keep using tube

I broke off the key guide on a black glass Sylvania vt-231 and a black glass RCA while using a Garage 1217 tube adapter. The key hole is very tight on these adapters and so are the pin sockets. I know the tube is still good just don't know how to orient the pins withe correct socket holes. If I can figure this out I can just use a paint pen on tube and socket for the future. There are no numbers in the bottom of the tubes so where do I go from here? I have more of these tubes but hate to throw in a junk drawer. Thanks for any help.
Line up the mark where the key used to stick out with the slot in the tube socket. 
Thanks vinylzone and millercarbon,  on rca vt231 you can see where key may have been on the other tube not so much. If my memory is right I think my Mullard fx2's have numbers but none of the 6sn7's are numbered. I even pulled out some matching tubes but there is no consistency in orientation of keys and plates. I may have to junk the Sylvania black glass if there is no way else to tell. Any other ideas, thanks for trying.
If you have the original key from that tube it should align to the tube like a puzzle. IOW, there should only be one way to attach it back. At least that was the way it was with my KT88 that had the same  damage. However I can see that the 6sn7 would be more challenging. Mine are all stamped the same BUT they are new not NOS. If you cant be sure the its cheaper to chuck it than repair the amp from a defective tube
I've done this.

With strong light, magnifying glass, careful look by young eyes, see where the positioning tab was broken off the tube center post. Scratch a vertical line in the tube base where the tab was. Line up the scratch with the key slot in the tube socket, insert.

The whole center post snapped off? Still have the piece? Figure out where the tab would be if the broken piece was re-attached, scratch the outside of the base like above. 
You can use a keyway replacement from Tube Depot,

The filament pins of 6sn7 are 7 and 8, you can use a DMM ( digital multimeter ) to identify the filament pins by using ohm meter, other pins will show open circuit except pin 7 and pin 8 have resistance reading.

Thanks imhifan, got it, put meter on pins and found 7 and 8 and key is located between 1 and 8. Your awesome thanks for the info, you saved some sweet old vt-231 tubes.