Difference in SC distributors

[bones]

I have temporarily installed an '81 distributor in my '78 car with a vacuum advance /retard mechanism as the retaining flange on my purely mechanical one broke during the engine refurb'. The car seems a little slow to build revs above 4,000 rpm when normally it takes off, does anyone think the distributor could be the issue? I have been told an '81SC distributor has significantly different electronics and mechanical parts to an '78SC distributor can anyone confirm this?

 

[bones]

I have the answer so to add to evertone's technical knowledge here goes.
The early 77/78/79 SC s had a purely mechanical advance (bob weights and springs) This system makes up the total advance. On 80-on SCs with advance only - the very late SCs have vacuum advance and retard - the mechanical advance curve is limited the rest being made up by the vacuum. This explains why this distributor would not work in my car without the vacuum system attached, I was told it might, so as they are in short supply I thought it worth a punt. The solution now is to swap the inards from the early model to the later body and blank off the vacuum.