Rubber bore plug ID help

[leadfarmer]

What are these for? Packaged 1953.

Plug side is tapered and measures 72mm to 74.5mm, too small for 3”.

[Quest Master]

Clarity on measurements, please. The base is tapered, is it 72mm (minimum) to 74.5mm (maximum)? What is the diameter of the top (cap). If the plug portion is 74.5mm (maximum), that would make sense for a 75mm gun, as it would have to fit inside of the lands and grooves (rifling) inside of the barrel. Also, since it was packaged in 1953, it may have shrunk a smidge since it was produced, so the measurements may have decreased a little.

[leadfarmer]

On the plug side, it is smallest at 72mm, call that the inboard face, and it tapers up to a max of 74.5mm where it shoulders against the cap. I wish I had a 75mm to try it out.

The package calls it a blast tube plug. I believe blast tube is an aviation term for tubes around gun barrels mounted in wings.

[Quest Master]

The first line says FJ-2, which may mean the North American FJ-2 "Fury" aircraft. This aircraft did have 20mm Colt Mk12 guns mounted under it, with with a much larger muzzle. And NA (North American) could be part of the part number listed. Additionally, the North American NA-181 Project, was the FJ-2 "Fury". Just a WAG.

[leadfarmer]

Well there's been no break through in this mystery.

I'm going to sell these plugs for $15 a piece if anyone wants one. That's shipped in the US 48 states.

Send me a PM if you want one or two. I have about 12 available. Maybe more.

[W. Winget]

A 2.75" rocket comes out as 69.85mm, so the launcher would be ever so much bigger for holding the rocket...
So perhaps they are covers for the back of the rocket pod. Plenty of pictures out there so no need to illustrate here.
V/R W. Winget

[scottyowl]

These are for a 40mm bofors flash hider.

[Quest Master]

The first line says FJ-2, which may mean the North American FJ-2 "Fury" aircraft. This aircraft did have 20mm Colt Mk12 guns mounted under it, with with a much larger muzzle. And NA (North American) could be part of the part number listed. Additionally, the North American NA-181 Project, was the FJ-2 "Fury". Just a WAG.

Definitive and final answer:
Contract NOa(s)51-642-i (third line of text on package) is the government contract number with North American Aviation for the NA-181 Project, FJ-2 "Fury" Navy/USMC fighter aircraft.