SaxonPig, I'm afraid that your pressure numbers are incorrect. According to SAAMI, the MAP (maximum average pressure) of standard pressure .38 Special is 17,000 psi and .38 Special +P is 20,000 psi.
http://www.saami.org/specifications_and_information/VelocityPressureData.pdf
So, this means that any .38 Special ammo generating MAP less than or equal to 17,000 psi is standard pressure while any .38 Spl ammo generating MAP of at least 17,001 but not more than 20,000 psi is .38 Special +P. Given this, your 16,500 psi figure for Winchester standard pressure ammo seems correct as that's 500 psi below maximum as does the 18,000 psi figure for +P as that's 1,000 psi above standard pressure MAP but less than the 20,000 psi +P MAP.
As to the "hotter" ammo of yesteryear, I'm a bit skeptical of old advertised velocities. The ammo manufacturers have been made more honest over the years as chronographs have become more accurate and more widely available. I suspect that many of the older advertised velocities that you cite were overly optimistic, taken from long test barrels, or both. Likewise, pressure testing equipment has improved and I suspect the pressure of many older loadings turned out not to be what the manufacturers thought they were.
You are probably partially correct in that ammo makers and loading manuals have become more conservative over the years. In our litigious society, people simply cannot be trusted to understand that "never exceed" is a rule rather than guideline or that "modern guns in good condition" excludes granddad's no-name Spanish S&W-copy.
As to the OP's original question, I can only speak to S&W and Ruger as those are the brands I'm most familiar with. Officially, S&W rates any all-steel K-Frame made after the introduction of model numbers (1957 or 1958 depending on the source of information) as OK for +P ammo and all J-Frames made after 1999 (introduction of the "magnum" J-Frame). While S&W is silent on the subject, any .38 Special N-Frame in good condition is also just fine for +P ammo (.38/44 Heavy Duty and Outdoorsman later designated Models 20 and 23). Any Ruger chambered for .38 Special is also fine for +P ammunition.
Personally, I think that S&W is somewhat conservative in these recommendations and I would classify post WWII K-Frames and pre-1999 J-Frames as OK for
limited use of +P ammunition so long as they are of all-steel construction. "Airweight" models including the Models 12, 37, 38, and 42 are best fed standard pressure only.
I am not familiar with Colt's recommendations on the issue, but I personally would treat them the same as vintage S&W's: post-WWII steel-frame guns are OK with
limited use of +P ammo while aluminum frame and pre-WWII specimens should be relegated to standard pressure only.