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It is just John jokingly donning Jimmy Saville's costume of the Abominable Snowman during the rehearsals. Nothing special.
That's the Yeti costume that John was wearing during one of the sketches. Only John had the Yeti costume. There's a story that during that time John was buying a car and the car dealer was informed that he has to get the cars outside the Hammersmith Odeon, because John didn't have time or patience to go and buy one himself, and the car dealer was shocked to see Yeti Lennon looking all models and finally buying one.