I haven't paid as much attention to the 6 pack duals as the hemi, but some interesting stuff in the presolite book and mopar service manual. Some errors/omissions too.

The Early 6 pack Manual was 4014C. That would have 30 mech at crankshaft with about 22 max vacuum. Initial is 5BTC

The 70 Service manual seems to have an error and the manual shows wrong distributor and data is same as Auto (Auto matches Prestolite). I think your 70 Manual should have 30 mech and 5 or less Initial.

You should only have about 25 total at 1900 which you see. But you get the rest or 32 to 35 total at 4800 rpm. Vacuum plugged. Have you reved that high for total?

The 71 service manual which has the tune up data and matches the Prestolite shows a IBS-4019B with 11 mech max. With Initial at 5BTDC. Really interesting.

The 4019 and 4019 A do have 18 max mechanical which seems to match what you are seeing. The 4019 have vacuum max at 19deg.

The 71 had a flat curve too. All in at 1900rpm from the service manual at about 27 deg total.

I can't explain why the 71 six pac total advance is so low.

Getting confusing huh