Yeah, lots of old lore still being spread around like old wives tales, even amongst those presumably "in the know." One thing I learned a long time in other endeavors -- not just pool -- is that people can be experts in the field and still carry long-held myths and misinformation forward forever. These guys are top players, not scientists or business people, and a lot of what they "know" was repeated to them by someone else that was misinformed.
Scott Lee could chime in here, as I recall he talked to Skip not long ago about this and Skip confirmed yet again that there is ZERO difference between pre- and post-flag Masters. The flag had nothing whatsoever to do with formulation, and was strictly a 9/11 tribute. And apparently the flag doesn't even go on every cube they produce, I've heard they still produce some chalk in no-flag packaging. So some of the "pre-flag" you think is old might actually be newer than some flag chalk.
There was some speculation that the "difference" was in lead content, that Tweeten wanted to quietly remove the lead without publicly admitting its presence and used the flag as "indicator" of no lead content. Skip also quashed this rumor by saying there were trace amounts of lead in years past in certain colors, but it was a miniscule trace that had been eliminated years before the flag came about. And since blue is by far the most popular color of Masters, likely wouldn't have ever had lead in the first place for the reasons someone else stated above.
It's certainly possible in some cases, but as noted above you can find newer "no-flag" chalk as well, so it likely doesn't explain all the cases.
When people get an idea in their head, it doesn't take much to "convince" them that the idea is fact. Billiards chalk, at least among those that are roughly similar and not total crap or radically different, is something that's still largely subjective to a person and difficult to articulate consistently. Without being put under a microscope and evaluated under laboratory conditions, people are still going to convince themselves that it's better/worse/different than something else based on some pre-conceived or misunderstood bias, regardless of whether there's a factual basis behind it.
And that's what keeps competition thriving, which is okay.