I'm pretty sure that there was only one version of Letter Go. It was only issued by Qubic Games while they were in the Springfield location.
It uses the same size box, and the same game board as the more famous Qubic game. The game boards even have "Qubic Games" and "Springfield, Mass." on them.
The letters probably came in a punch out sheet that was inserted into the box. Some of the cardboard letters have noticeable ears that would have been from such a punch out sheet.