Wasn't the Captain Midnight decoder shaped like a jet/spacehsip and hollow so you could pour a drop of milk in and write invisible messages with the removable nose?
As for Tom Terrific and memory, this from central Norway and most of my life later
I can be a plane up high
A diesel train go roaring by
A bumble bee or a tree
When there is trouble
I'm there on the double
From th'Atlantic to the Pacific
They know Tom Terrific
As for the baking soda subs, my Dad used to use a deodorant that came in plastic squeegy bottles (I remember them as green and the sound of wheee-juh, wheee-juh, two doses under each arm in front of a misty mirror in the mild panic to get ready for work/school)anyhow, I'd get the empty bottles, filled with water in the bath tub I used them to strafe the subs when they surfaced...
I should really get back to work...