I have, as of 05/02/2025, received more information about these alarms. It appears that this specific type of flush plate was manufactured by a different company, possibly Simplex, and used by Wheelock for use with their 3x series horns.
Picture for reference:
Courtesy of: @Fallen_Galon
According to a post from the said user above, featuring one of his alarms from his collection (seen above), these also came with Simplex 4048-X Horns. This means that all mounting accessories presented in the advertisement previously mentioned here could also be manufactured by that company and come with the 4048-X as well. I do not know the specifics, just that one was spotted on a 70s Wheelock ad, at my high school with some 3x series horns, and with a 4048-R horn in the said user’s collection.
Also, I now have better quality pictures of both said alarms at my high school:
