I cannot imagine anyone manufacturing a firearm with an unmarked safety. Or a manufacturer's liability insurance provider allowing an unmarked safety.
The Sten selector is originally marked "A" and "R" for automatic and repetition. Push A for auto, R for semi auto.
As you know, Spencer converts the selector into a safety. Rotate it one way, and the tripping lever is cammed out of contact with the sear, and pulling the trigger will not fire the gun, rotate it back, and the tripping lever can engage the sear. This is no doubt one of the features he intends to patent.
A selector marked A and R which would normally be pushed left and right, but is actually a safety which has to be rotated to engage, but which is unmarked for the safety function would be a lawsuit waiting to happen. Not only should it be marked, it should also be detented, so it cannot be accidentally disengaged.
I have a friend is a retired firearms industry engineer and designer who has testified as an expert witness in over 40 firearms related court cases. I'll run the scenario past him for his opinion.