[Roadsters] distrubitor

[Dana Scribner]

This being Memorial Day I hope everyone has taken the time to thank those who
gave their lives  so we are able to live ours as we do.  I have been having a
problem with the idle on my 1969 2000 SU carb no emission car. I can set it
then go for a ride and when I stop at a light the idle keeps dropping off it
100 or less or it stalls. At high idles it's fine. I gave up and took it "The
Guy" wit 40 plus years on SU knowledge.  Long story short he thought is is a
distributor problem. I opened it up and there it was a broken spring sitting
at the bottom. So this leads me to my questions. On the timing lever plate it
is marked 7.5; I looked at my supply of parts and some are marked 17.5.
Why?Are the weights different on the 2 distributors? How do you tell them
apart?Are the springs different between  the  one marked 7.5 and the 17.5
distributor? How do you tell them apart?Where the vacuum advance is attacked
to the housing I see the a 17.5 stamped in ink on the 17.5 marked timing
lever. Are the 7.5 ones not marked?is the vacuum advance different between the
2?  Dana S1969 2000