I must be reading a different book than what you folks are. The 10-7 is a narrow barrel and the 10-8 is a heavy barrel; they complement each other. The 10-7 started in 1977 and went until the 10-9 in 1988. Notice the 10-10 (heavy barrel) was also introduced in 1988.
The three letter prefix began in the transition years of 1980-1983. They did not use them in any order. I have a 10-7 that shipped in Dec 1983 with a s/n of 18D3529.
and here is a 1981 10-8 (shipped in Dec 1981)
Notice neither barrel is pinned.