Brian,
I didn't see a 638 on the site I was perusing nor did I get anywhere on Loctite's site (?). On a hunch, I opened up a Google query for Loctite 635" since a Loctite "35" is referenced in the TC shop manual supplement. Turns out, a Loctite 635 retaining compound does exist and is available.
Here's one description: "635 Retaining Compound is a high strength, high viscosity, single component anaerobic retaining adhesive for cylindrical joints. it cures when confined in the absence of air between close fitting metal surfaces. 50 ml bottle. This product complies to MIL-R-46082 TY 3...."
It is also green in color, however it has a slow cure to allow fitment of parts so it sounds like something different from what you have. This 635 stuff is expensive, I think a 50ml bottle runs 40-50 USD. Handles temps up to 300 deg F. The Loctite product number is 63531.
I think this may be the stuff I'm looking for.
regards
Rick