What the heck is an R10 or V10?

Starting in 1988 GM began rolling out the new GMT-400 body style trucks and they had the C/K designation followed by an updated "1500", "2500", and "3500". GM decided there couldn't be two trucks with C/K designations so the Sqaurebodies from 1988 to 1991 were given the new R/V designation.

From 1988 to 1991 Chevrolet and GMC sold the new GMT-400 body style trucks alongside the Squarebody body style until 1992 when the Squarebody was no longer available.

This was a confusing time for GM as the Blazer and Suburban kind of just awkwardly converted to the GMT-400 platform by just kinda copying the front clip until 1992 when they finally fully joined the GMT-400 platform.