Inside the transmission, I recently noticed that my reverse band adjusting nut does not have a notch in it with an accompanying lock washer as does the brake band to avoid the nut backing off by itself.
Is this normal or did the previous owner lose the lock washer and replace without it?
Please advise...!
Not correct. Change it out with the correct washer and nut.
Don't re-install it. Make sure you have the notched nut with the correct washer. A nut and washer that size will really give you a bad day if it winds up loose in the transmission. Make sure the washer has the locking tab on the bottom that slides into the grooved side of the pedal shaft.
You might also run a magnet down under the trans. to see if the original washer is still in there for safety sake.