... a comment regarding standard 'philosophy of use' for vacuum advance types.

       The early U.S. and U.K. vacuum units were configured to advance the timing when cruising which gives better economy and lower running temperatures. There have been (at least) three variations on the theme used to control emissions and pinging (some may call it pinking).

1. Vacuum advance was reduced at idle (to nothing) and when accelerating (depending on throttle opening) to eliminate pinging. This configuration picked up its vacuum from a tap directly on one of the carbs, thusly called 'ported' vacuum. This scheme usually had vacuum actuation that started to advance around 5-7 Hg/in. and would give a max. of around 10 degrees of vacuum actuated advance.
2.  The second major type usually started to advance at around 7-10 Hg/in of vacuum, giving a max. of around 5-10 degrees of vacuum advance. Most Smog equipped engines (later years) used this advance, where vacuum was applied at idle and reduced when accelerating.  The vacuum pickup was tapped directly from the intake manifold. Manifold controlled vac. units were usually used on distributors which gave a much greater total mechanical advance, then those using ported vacuum.
3.   The third was an intermediate type. Kind of a mix between the first two. The manifold fed the vacuum advance unit, with the advance curve only giving around 5-7 degree maximum.
and didn't start to have any effect until you let off the peddle a bit or were idling. These units were usually used with a distributor that mechanically started to advance at a much lower rpm range.

     This started as a MGB only list, but I've found that the MG Midget and Triumph Spitfire all use(d) the Lucas 25D4 distributor at one time or another. Therefore those distributors are now included (where I could verify the info) as well and are identified where appropriate.