You are correct. That is a Mellotron. A Mellotron is an early sampler, it is with no comparison to todays digital samplers. This instrument was very poor and went out of tune at the drop of a dime. A few people I know have played one. John Paul Jones was another that used one (ex. The Rain Song - Houses Of The Holy)

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"I mean they're Gonna Kill Ya, So like, if you give em a quick shh short, sharp, shock, they don't do it again, dig it..."