On the real Armstrong turntables I’ve seen, there is a plate that the crew can slide across the gap to keep the rails aligned. I see the manual indexing of my model as an opportunity to replicate this activity, as well as providing positive alignment for the turntable.
When I originally thought about this, I figured the lock would be a pin and socket arrangement. Because the wheel drive is mounted on a plywood plate beneath the lazy Susan, the sockets need to be quite low so they can clear as they rotate over the drive plate.
Until recently, I had been envisioning horizontal sockets with a square pin bent to the level of the PC board. This would mean that the gap between the end of the guide for the pin and the socket would be large, and that leads invariably to slop. Then, I thought about vertical sockets and some sort of lever to drive a pin into them.
On my way home the other night, I finally came on a solution that I like. The sockets will be neither horizontal nor vertical, but a shallow angle. This enables a straight round pin driven through a nearly straight choke cable. Indeed, if the angle is shallow enough, I may not need a choke cable at all, but perhaps a solid rod will suffice.