GM replaced the point distributors in 1975 with HEI distributors in all of their vehicles including the 250 6 cylinder. It appears that the PO installed one in your truck. It's a common upgrade, if it is installed correctly, and it works OK, leave it alone and drive it.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine