I have some vague recollections of riding the K train to the Museum of Natural History with my parents when I was about 4-5, but I don't recall the slant cars. I remember the flat-front versions (R-40M and/or R-42). I can't remember if they were the old interiors or the rebuilt ones. I even remember a Redbird (R-27/30), both on the K and on the C after the C took over. I remember the takeover - K signs were still up all over, but the K train was missing from the new map. My father thought it was a misprint! That was until we found out after waiting for a K that never showed, my father asked a transit cop, and found out that there was no more K - we had to take the C.
The first slant-front R-40 I saw was on the B, circa same as above. We were on the F train pulling into Stillwell Ave and I saw the slant-40 B train a couple tracks away and I was transfixed.
All throughout the CI visit, I bugged my father with "Can we please take the B train, I want to ride the B train!" I was expecting an exotic experience, but I was very disappointed to find out that the Wierd Train looked like any other subway car on the inside.