the parts manual is here: http://www.planetarypinball.com/reference/partsmanuals/BLY_Parts_1976/index.html#/90/

looks like it's the trip relay armature is what you need for an interlock relay.

looking at the parts manual I think it's part # P-7469-7 which isn't listed on pinball resources page http://pbresource.com/relay-BLY.html but would call Steve Young there to make sure.

actually FIRST I would check under the bottom board to see if the original plate was there.

the spring appears to be the natural color part# SP-199-28

if it ain't under the board I'm not sure if this one will work: ebay.com link: Bally Captain Fantastic Pinball Machine Game Over Interlock Relay Switch

or there is the "EM parts thread" https://pinside.com/pinball/forum/topic/the-em-seeking-parts-thread