Hey Fred,
AFAIK it is a stock spring. My G19 was a factory refurb so for all I know the former owner (a policeman in NJ, they gave me the original receipt) could have replaced it somewhere along the line but it looks to me like a stock spring.
I found out that my G19 wouldn't handle the "light" loads in the worst way. I had gone to a week-long training class and my S&W M38 Airweight locked up on the second day of the shooting portion. I had brought my G19 as a backup but I didn't have enough ammo for the class (1,000 min for the whole week). I bought some of those 4.5 grain reloads off my buddy (they work fine in his Taurus PT99) and when I get to the class the next day I find out the rounds won't cycle my gun!
Ended up buying factory rounds to finish the class.
Justin