Architectural Mailboxes 7540B-10 Mailbox Adapter Plate, 4" x 4", Black

Will this plate work with the Oasis Jr. Support arm?

Will this fit 6400p-r-10 mailbox? 11.3 x 18.2? (It's recommended by Architectural Mailboxes, but doesn't look like it will fit.

The post and hole in the adapter plate are 3.5 x 3.5. I cannot get it onto the post. Advice?

Would this fit the Gibraltar extra large rural mailbox ??