Indeed, plenty of famous art pieces have been 'fixed' and if you really wanted to revert a statue back to it's original form removing the number plate would be an easy job and do wouldn't take away from the real artistic parts of it. As they were placed on the flat base of the pedestal, not touching the primary artwork which is the statue itself.

Sure they could find far better solutions, and very likely would today. But I don't think this is a particularly big deal that it was done in the past.