Ammeter to Voltmeter...who does it?

Ok great analogy on the stream bit, very easy to visualize.
I agree. Well explained.

On police package cars chrysler did just that. Drilling 2 holes in the firewall and installing rubber grommets then running the leads through the firewall bypassing the bulkhead connector. Police cars had more powerful alternators required to run the lights, and radio gear. Chrysler knew this was a weak link with a 120A or greater alternator and so bypassed it at the factory on police vehicles.

Close. What I've seen on the older systems is one grommet with the ammeter feeds. This wasn't police per se. It was for various applications that were getting the 59, 60 amp Chrysler or any of the Leece-Neville alternators. There were various flavors of this scheme, but the idea was the same in all.

As you describe, the concept was to split the stream. On some it was done on the engine side, using a single post junction block. On others this was done on the ammeter's alternator connection (stud).

The ones with the junction block continued to use the standard bulkhead connection for the alternator feed.
By that decision, it appears the load they were most concerned about was the recharge of the battery. This is where on some configurations we see 8 or even 6 gage wires and a 10 or 12 gage fusible link.

Here's a Leece-Neville one from the '73 Dodge FSM
upload_2020-3-29_16-18-37.png

Skim through this to see another way it was done. In this case, they didn't want to use the bulkhead feed at all. Probably because that defroster grid pulls alot of current just by itself.
How rare is electric rear defrost?