Pimentel fought quite a lot of fighters with losing records (and incomplete ones I'd wager) and didn't face some of the better contenders of the day but his reputation as a puncher was legit I think. He stopped the very tough Mimoun Ben Ali as Al mentioned earlier I think and was approached with great caution by Olivares, Medel and Chucho Castillo (despite really being faded against Ruben too). Plenty of scary punchers don't always topple formidable or tough opposition, its sometimes just about how they crush the lower level types effortlessly Imo, which Pimentel did.
He's says in the interview that he KO'd Ben Ali but as far as I've heard he actually stopped him on cuts. He clearly was a puncher but it seems that he didn't have many if any against the top fighters.
I think he was the type of fighter that if you got into a punch up with him you could come unstuck unless you're someone like Olivares and he even he was tentative in the early rounds until he'd had a good luck. It looked like he stunned Ruben in the third i think but Ruben opened up on him then. Ray Asis was a decent fighter and held a win over Joe Medel in the Phillipines. Joe knocked him out in the return. Dad fought Asis as well and scored a shutout.
There is a Mexican newspaper archive, I think it's called herometica, something like that. I'll see if I can find anything on his earlier fights in Mexico.
That site doesn't seem to be working, shame because I've found stuff on there before on Castillo and Olivares early fights