My understanding of the old manual " lock hubs" is ...
They just disengage the hub from the axel on that side only.
Hence why you have to lock both individually.
Designed for less wear and tear unneeded on the front diff centre.
If you are in the old rear wheel drive mode, your front diff won't be turning over. But idol.
Locking in the front hubs in rear wheel drive only mode only, I think will just spin the diff centre .
Your front diff might be just an open diff centre, or LSD, , or remote diff lock.
It is good practice to engage them to help lube them occasionally.
All wheel drive mode varies as well.
This can make one wheel at the front work the same as ONE of the rear wheels.
Depending on if you have LSD in back, + / front as well.
Center diffs can offer full lock ratio ( front and rear shafts turn locked in at same speed/ratio.
Or in all wheel drive mode, the front usually get 45% of the power, but has the ability to clutch itself to various revolutions, depending on turning etc.
Early 4x4s only offered a centre diff lock, that kept the front and rear drive shafts locked in sync.
And had nothing to do with diff centre locks or LSD capacity.
If you had it locked up in the front, and in 4x4 etc, you would of found it very hard to turn on bitumen, sealed roads etc.
Heaps of combinations regarding 4x4 drive trains hey.