Vacuum advance

When brought my new engine, it came with a genuine Zenith carb attached.

It was a 24v military recon engine. I was using it in my 12v 88" so distributor, starter motor and new alternator went on.

I used the genuine Zenith carb on that engine until recently when I decided to put a new "replacement" Zenith that I had knocking about on it to see if it made any difference to the fuel accompany.

I have just noticed that the vacuum advance hole on the old genuine carb is sealed up?

How did the engine run so well when the vacuum advance was sealed up?

Would a 24v distributor be any different and not need this?

I am having problems when the throttle is blipped, the engine chokes then picks up. I was wondering if I should put the original genuine Zenith back on, but what about the vacuum advance?

What do you think could be causing the engine to choke when the throttle is blipped?

To much air all in one go?

Some engines have the vac advance tube going into the fanimold, maybe the carb you mentioned is for one of these.
If the engine dies when revved then if it just dies its short of fuel which might be the throttle pump setting within the carby, or if it tries to rev up then dies its too much fuel.