I actually did a fair amount of research on this some time back. The "para" stamping on a stem simple means that the latex used in the vulcanite formulation was extracted from a Para rubber tree ...
... and not from an alternate (and historically inferior) source ...
The distinction was an important one in the late 19th and early 20th centuries when, as the clipping above states, many rubber products were produced using the inferior sources. You can still find the stamping on some stems made in the 60s, 70s, and maybe even the 80s, but the distinction means less since the tree had been well cultivated outside of Brazil at that point and the supply of para latex was plentiful. I think some pipe manufacturers just thought it sounded cool. So, they kept using it.
"English" para refers to rubber that was produced from latex extracted from trees grown in the rubber plantations of British Malaya. At one time, these plantations were the largest cultivators of the Para rubber tree outside of South America.