At that time, it was usual for mascots to be stolen almost all the time and indeed, THETA was lost at Morphy Day that year in November. He was regained about two weeks later from the middle of the Round Pond in Kensington Gardens, where Guilds had floated him.
Nowadays, if THETA is stolen (which hasn't happened in 12 years!) then a new one is made so that each version is a 'Virgin'. This was not so in the 50s and 60s and THETA MK I suffered many thefts during his 6 year life.