Popping out of first - still play after replacing shifter and coupler
 

In our journey to sort our ‘81 SC (100k miles) that we purchased 18 months ago, we have been working to make the shifting better. Here is the sequence:
1) About 9 months back put in a Rennshift. Made things so much better but still had some forward / back play. So ordered a coupler, however did not install yet. Overall - shifting felt really great!
2) About a week ago it popped out of first leaving a stop. I thought maybe I just did not have it in gear and seemed like a one time thing. Then yesterday driving the car it popped out every time I started from a stop - not under hard acceleration.
3) because of it popping out of gear, I installed my coupler to see if the front to back play was causing/contributing to the first gear issue. Front to back play got better, but as you can see in this video there is still quite a bit of play :(.

https://youtube.com/shorts/ZM0l8LapCEg?feature=share

Anyone know of another easier thing to check, or is this tranny getting cracked open?

Dog teeth can break off and then the shift sleeve has nothing to engage with. Then the syncro ring, brake band and stops can come loose and go through the gears, worst case. Best to drain the gear oil and look for any small teeth in the magnet. Might be your adjustment though.

Thanks John!

It was happening before the adjustment. I installed the coupler / adjusted after the issue hoping it would make it better (and it has not).

More thinking your other reasons may be at play now.

Shift fork can also move along the shaft, such that the synchro cannot be fully engaged, and it pops out under load. Cause can be a hard shift into 1st in your case.