Here is Bompensiero's own info:
Earlier on the same page he says he began these visits to Tijuana around 1950 and this encounter with Colonel White occurred sometime after that. Says Colonel White commented on Bomp's promotion only a few days after it happened which led to the Moretti suspicion.
So it looks like the promotion happened in either late 1950 or 1951 given he says it happened immediately before the Moretti conspiracy unfolded. I lean toward Bomp's story given it was his own promotion and he was recalling it a little over 15 years later opposed to 30 years in Fratianno's story.
Here is another FBI interview where he tells the same story:
Note that he is referring to Scozzari not as the former underboss circa 1951, but the former underboss at the time of this FBI report in the late 1960s. Roselli and Scozzari attending the meeting is interesting given they were not on the admin at the time.
You can see they blocked out the word "caporegime" but it's obvious what it's referring to when it says he was "appointed" to a position a few days earlier and White could have only known from an informant. You can see it was even placed in quotes, which is common when they use organizational terms in FBI reports. Guessing they blocked it out to make the informant more difficult to identify given it was well-known Bompensiero had been a captain then.