I was under the impression it was the real gun and not a mock up. The original was recovered off of the wreck of the ship that sank in Matagorda Bay. I think the spelling of the ship is the Helen Tooker. The Spaniards took the gun to San Antonio. (Texas beloved to Spain at the time). The Mexicans took it from them and The Texicans took it from the Mexicans .
Yes, yes, and yes. You are precisely correct about that gun, and the illustration is of that gun.
That gun is original, and was indeed apparently taken from the wreck of a privateer in Matagorda Bay from when the pirate/corsair/privateer Aury was backing Xavier Mina in 1817. It is thought that that gun was on the northwest bastion, not too far from the north wall where Travis was killed early in the final battle. The gun carriage is of a basic "field type" but it did put the gun at an exaggerated height for display purposes so people can't try to climb on it. (guilty confession: Once, while visiting 1898 battlefields in eastern Cuba, I scaled a monument and hopped on a cannon for a photo opportunity at El Caney...)
The modern reproduction gun is at a temporary exhibit that will eventually be removed/ taken down. It's at the site of the southwestern bastion, while the original gun in your photo is near the main entrance, the Shrine of Texas Liberty/ old Mission chapel, and the Long Barracks.
The thing is so heavy that the wheels of the carriage have to be rotated periodically to avoid a "set" in the axle and other parts of the carriage. The carriage was hand made by an outfit in South Dakota, which caused some issues, because when the gun tube arrived from Ohio, there were variances in where the trunnions were, along with other now-forgotten "fitting" issues between gun and carriage.