Older manuals also tend to be hotter overall than current ones.
I'm wondering if that is just the "lawyer" thing.
It is, to a degree, and its also a lot more.
Each gun and ammo component combination has its own traits. And while generally very similar, (which is why we use published data as
guidelines) they can be much different.
Powders may have changed over the years, today's H110 might not be the same as it was 40 years ago, for example, and loads get adjusted for this.
Measuring pressure is different today than it was a generation ago. This is, I think one of the bigger factors.
In the old days, pressure measurements were, essentially, "by eye". It was a comparison method, comparing observed results in certain places (case expansion, primer flattening, etc.) against a "known standard" , (for many years the "crusher" system copper, or lead), and determining that X amount of change = xx,000 units of pressure.
Industry standards (SAAMI) were developed this way. Now, we have computers and other higher tech methods for measuring the pressure directly and more accurately. And there have been a few surprises.
And this is where the lawyer factor comes in...
(pressure numbers used are for illustration only)
For example,
Grandpa's .38 load was 17,500. We all knew this, for generation.
The SAAMI spec is 17,500
now, surprise, the computer shows that what we all thought was 17,500 is actually 23,800.
The lawyer steps up and says, "that's above the limit, it's not safe, we are liable!!"
DO they raise the limit to what it is now proven to be? Which we have been shooting safely for generations?
NO.
They lower the load to meet the old established (and now shown to be inaccurate) limit.
After all, it doesn't "hurt" anyone, and it covers their butts in legal matters.
Just a theory, and worth what you paid for it, but I'd be surprised if there wasn't some truth to it.