Holley Loadamatic distributors were a vacuum only advance system that basicly were load sensative system. They used a carburetor with both a spark port at the throttle plate and a venturii port. Many of them had a spark valve on the side of the carburetor. They are easily identified by the very large diameter vacuum diaphram and two springs on the breaker plate with no centrifugal advance mechanism. If you look at the top side you can see the springs are angled differently and different strengths. The light spring controls the venturri advance and the heavy one the port spark or no load advance. The curve specs on these will have a very low vacuum value for the first part of the advance curve on the order of .3 to 1.5" hg. Ford used them on the 49 - 53 flat head V8 and 52 - 56 Y blocks and 49 - 51 Flat head 6 and 52 - 67 OHV 6s. A bad vacuum advance on them would kill both fuel economy and performance.