The #75 is likely a reference to Batman's 75th Anniversary, which is being celebrated at various events throughout the Con. Though, this does beat the photo he sent out yesterday.
One of the items Warner Bros. has on display inside the San Diego convention center is the Batman cowl and cape worn by Ben Affleck in the highly-anticipated Batman v Superman: Dawn of Justice. Take a look at the photo below, courtesy of Twitter user Dan Didio.
Batfleck costume is in the house! pic.twitter.com/wJCYm4S1ne— dan didio (@dandidio1) July 24, 2014