Hit the "replacement for" link and a Model A teacup pops up. You might want to ask them to measure the diameter for you before ordering it.
Even if it is a perfect fit for the Hall manufactured 26-27 accessory stoplight, the originals didn't have STOP cast into the lens. STOP showed through the lens because of a metal plate with those letters stencil cut into the plate, but if you don't have one, anything is better than nothing.
The tuna can 27 to 29 style Model A lamp while smaller then the Hall lamp did have the metal stop stencil. The tea cup style is much smaller and the oval tail light in that replacement looks bigger then the Hall version.
Same lens at Snyders;
The lens shown in your link is both over priced and for a 29 to 31 Model A tail light.