The 135 may be different, but the standard Fender arrangement used a switched main speaker jack which shorted to ground when there was no plug inserted. This prevented damage that would occur if the amp was used with no load. It also caused consternation if you plugged into the extension jack and not the main jack, as the amp wouldn't work. We didn't know at the time why they did that, but you didn't have to make that mistake very many times before you learned not to do it.
[b]Edited by Bill Fitzmaurice, Today, 10:53 PM.[/b]
Same arrangement on the current Bassman 100T