Yes, that's an old SS-SS2-A.
is what you'd use to go from the tube fittings to 1/8" NPT. Any manufacturer that makes double ferrule compression fittings will have an adapter like that.
Also, that Harris 25-100 should work fine. You just need to remove the old nipple and nut and replace it with whatever cylinder fitting you use there.
edit - you'll likely need new ferrules. SS-200-SET