From the grammar point of view...I can't understand it:-)
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I heard Pancho, (The Cisco Kid's partner) saying 'Let's went' well over 60 years ago. I understood it to be a mispronunciation of 'Let's go', the meaning was certainly the same. (Fue/fuimos can mean Go or Went). I have often thought of it and have said it many times myself since. (Funny how it sticks). I did get one funny look from a person some I'm back but simply told them to look up Pancho and the Cisco Kid TV series.

As slightly similar thing crops up in the last verse of Marty Robbins' country ballad "Big Iron", released in 1959. Notice the past tense "went" instead of the expected past participle "gone"

It was over in a moment and the folks had gathered round
There before them lay the body of the outlaw on the ground
Oh he might have went on living but he made one fatal slip
When he tried to match the ranger with the big iron on his hip
Big iron on his hip

Students: Are you brave enough to let our tutors analyse your pronunciation?

You are RIGHT! "Let's Went...!" ... was a signature line of "Pancho", the Cisco Kid's trusty side kick on the old TV program,

Cobra Aqua 153 is correct. "The Adventures of the Cisco Kid" was one of several Westerns/cowboys series for children in the 1950's, along with Roy Rogers, the Lone Ranger, Lash LaRue, Gene Autry, Hopalong Cassidy, and Wild Bill Hickok. Leo Carrillo played Cisco's sidekick; he and his entire family annually rode in Pasadena's Tournament of Roses Parade. Many episodes concluded with the bad guys in the Sheriff's custody, the falsely-accused swain back in his lady's good graces, the stolen money returned to Wells Fargo, and Pancho and Cisco about to ride off into the sunset. "Aye, Pancho!" "Aye, Seeees-ko, Let's went!" (Epstein didn't kill himself)