My 700, serial # A6662xxx, produced in 1979, purchased in 1980, has the bolt lock safety. With the safety on "S", the bolt can not be operated.
I understand that there may have been a program whereby Remington modified the "bolt safety" rifles so that the bolt could be operated with the safety lever on "S", like the current rifles.
I have not had a problem with the bolt lock safety on my ADL. When I want to unload it, I point it in a direction which won't be tragic or embarassing if it goes bang, put the safety on "F", and then open the bolt to empty the chamber. I then operate the bolt far enough to strip the cartridges out of the magazine, but not far enough to chamber them, and then drop them out of the loading port into my hand.
After unloading the rifle and closing the bolt, I open the bolt about an inch to confirm that there is nothing in the chamber, then close it again. Then I put the safety lever back on "S".
Of course, if the action is a BDL. all one has to do to empty the magazine is drop the floorplate.
As well as the lock on the safety, older models do not have the rail "track" in it (can't remember the name) that makes it more anti-bind than without. As stated above, they have both 6 and 7 digit non lettered before lettered receivers came out.
I believe the anti bind rail that chad3 mentions was done when the A prefix started as well as the .017 change in the rear bridge height.
The first 700's in '62 started with a 4 digit serial number. I had a 9XXX in a 222 carbine and sure wished I wouldn't have hated the X so bad and sold it it hopes of selling the bad memories but I sure still miss the deuce.