If the gun was ever in Cuba.
I don't mean to be a jerk but...
The grips are from Cuba and are marked as such. But there are no markings on or in the gun itself that indicate it was ever in Cuba.
The gun was made by FN for the U.S. market. It could have been shipped to Cuba for some reason by FN or Browning but a letter saying so is needed to prove it.
To show it was from Cuba, or had spent time in Cuba you would need paperwork. A bill of sale, a receipt, a letter from the original owner tracing the gun's history, etc. Even a letter from FN or Browning showing that guns in that serial number range were shipped to the Cuban military or the police. Keep in mind also that most military or police guns are marked as such. This one is not marked.
It was purchased years ago as part of a lot of about 30 older guns, $1500 for the whole lot from an estate sale.
So "years ago" the gun was bought in the U.S. and had Cuban made grips on it. Not so unusual in some parts of the country. The folks who sold the gun told you nothing about it's history. They had no paperwork.
I was told it was carried by select Cuban military officers during the beginning of Castro's reign
But you were not told this by the seller of the gun and there is no paperwork with the gun to support that idea.
We took it to a smith in Florida who's father happened to be a Cuban refugee from way back. When his father saw the gun he got really excited and started speaking spanish so quickly his son couldn't translate fast enough. Which is where we got the little bit of information we have. He remembered seeing the officers with them along with some other details I can't remember
So the smith said he recalled seeing guns and grips of this type on some Cuban officers at some point. The gun looks like some guns the smith recalled from his youth and it made him happy to see it. The son could not accurately translate all he said and you don't recall all the son said. But the smith did not say, and did not show you how, your gun was from Cuba and had been carried by Officer's of the post Batista Cuban Government.
This is what I meant earlier when I said you buy the gun, not the story. Without paperwork or other proof a story is just a story. Maybe true, maybe not. You don't know and neither do we.
You have a nice 1959 P35. Clean it up and it will be a good shooter and an heirloom. Standard pot metal plated grips from Cuba and maybe the gun was there and came back.
tipoc