The lamps install from the rear of the dial, and were contained in the rubber molded plugs. These plugs also keep out the humidity in the atmosphere. At least they did when they were young and supple in early seventies.
If they are still intact today, they are now pretty hard bodied, if not cracked with age.
This would allow moisture inside them. To fog the inside of the lens.
To cure is a two step process.
Step One, is to the dry out the interior of the gauge, making it a higher temp than the outside atmosphere.
I have an oven with pilot flame inside it, that always keeps the temp inside higher than elsewhere, and this lowers the humidity inside the guage. It only about 20-40 degrees warmer inside there, even with the oven off. But, that is enough to dry out the air relative to outside to the outside world.
Step Two, is to take your heated gauge and install it back on the bike, insert those instrument bulbs/holders and have it seal up the gauge so water and humidity have a hard time getting inside the gauge.
No humidity, and the inside can't fog up with condensation.
You will have to take the gauge out of the bezel mount, but you don't have to dismantle the gauge itself.
I expect there are some process variation on the scheme using a heat gun/ hair dryer to gently dry out the inner atmosphere of the gauge and drive out the moisture. But, you will need to address those instrument plugs/seal on the back in some way. New rubber would be ideal, if that could be found.
Cheers,
