1971 Duster Wire ID..
If you are looking at the Chassis Manual Rally Instrument diagram, you are correct that fuse's connection is not shown.
We can be sure its fuse 'A' by checking the next fuse circuit, and it is in fact Black with tracer.
But we have some clues.
It's on the Accessory Feed coming from the key switch. So its for some option that required the key be in run or accessory.
The next page has some options. Scanning for a blue that connects to the accessory fuse.
Here's one, and it matches the connectors you show.
Looks like that may answer both questions.
If you are looking at the Chassis Manual Rally Instrument diagram, you are correct that fuse's connection is not shown.
We can be sure its fuse 'A' by checking the next fuse circuit, and it is in fact Black with tracer.
But we have some clues.
It's on the Accessory Feed coming from the key switch. So its for some option that required the key be in run or accessory.
The next page has some options. Scanning for a blue that connects to the accessory fuse.
Here's one, and it matches the connectors you show.
Looks like that may answer both questions.