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Check your Ref. Guys. I think they never used the tubes on the K model
According to Wikipedia, their basic information states that wing tubes were used later in D/K version once the wing hard points were strengthened.
Wikipedia: North American P-51
The wing racks fitted to the P-51D/P-51K series were strengthened and were able to carry up to 1,000 lb (450 kg) of ordnance, although 500 lb (230 kg) bombs were the recommended maximum load.[35] Later models had removable under-wing 'Zero Rail' rocket pylons added to carry up to ten T64 5.0 in (127 mm) H.V.A.R rockets per plane.
Joel