1973 450SE locked out of reverse

I have a 1973 450SE with 100k miles and ever since I bought it (over two years ago) it has had a slight two or so second delay before engaging when I put it into reverse. This morning when I went to back out of my driveway, I was stuck in some snow so I figured I'd rock out of it. However, once it was clear my car was free, it was no longer engaging at all when put in reverse. The reverse lights still come on, the seat belt buzzer still lets me know if my belt is on while the engine is engaged, but it does not engage at all. There is no grinding and the fluid levels are within range. So first is there supposed to be a lockout on this transmission? And if so, is it possible it is simply the lockout switch that is stopping the car from engaging?
Thanks in advance

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