David,
I'm certainly not an ignition expert but have had experience with Bristol engines and distributors over the past 40 plus years.
All Lucas advance curves are quoted in Distributor degrees and Distributor revs .
These figures have to be doubled to correlate with engine RPM & Deg advance .
There could have been a misunderstanding between Andy and the 123 literature as to whether Distributor or engine revs & degs were quoted.
Ask yourself "what would the manufacturers of 123 distributors know about what advance curves Bristol 6 cylinder engines require in their current mostly modified condition, without even considering the various types of Bristol engines produced".
I suggest you don't think too much about the curve until the rolling road tells you what advance curve is required for your engine.
Yes the camshaft , compression , fuel octane and other factors can change the advance curve required.
A vacuum advance is probably not necessary with any good quality distributor.
Geoff
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