adjustable vacuum advance
 

I tried my allen wrenches in the tube, but did not hit anything. Maybe not long enough? Are most durasparks adjustable?
Thanks,

I've never seen a DuraSpark that was adjustable. Adjustable vacuum advance units are generally an aftermarket modification that you install.

I've got two of 'em, one in my car and one in my parts bin. I got the one off a 78 Lincoln Versailles so I'd say it's a good chance it was stock. I've seen a few that aren't but I've had at least a half dozen that are adjustable so they do exist. I actually found this out from a guy at a speed shop when I went to buy an adjustable vacuum advance, he told me to try it first and because of that he lost a sale ;).

Oh yeah, if it is adjustable, you'll hit something with it only about 1/2" into the vacuum hose hole so it sounds like yours isn't adjustable. BTW the adjustable ones I've found have mostly been on 302's after 1978.

Okay, I've got one here in front of me. If you put too small of an allen key in it then it will just poke right through (I guess the adjuster is hollow) so use the biggest allen key that will fit in the hole and it is about 3/4 of an inch deep in the hole (instead of 1/2" like I stated earlier).
BTW, it's got MOTORCRAFT cast into it so it's definitely stock.