Spark control module

I found another '94 S10 to replace the one I wrecked. I was giving it a tune up and was unable to remove the spark control unit inside the distributor cap. .A 1/4 inch socket was too big and 3/16 was too small. I switched to metric and 6mm was too big and 5mm  too small. Does anyone know what size this stupid thing is so I can try to figure out how to remove it? Thanks, It's a 4.3 V6 fuel injected with the distrib at the rear of the engine ( That's to make it harder to reach)

GT