Looking at other sources for the VIN information today I realized that engine code B means my jeep originally came with the GM-151 "Iron Duke" and the SR5 four speed transmission. Wikipedia had the answer
From 1980 to 1983, the CJ-5 came standard with a "Hurricane"-branded
version of the GM Iron Duke I4 with an SR4 close-ratio four-speed manual transmission.
Looking at this Engine Code
web page again I see that
B- 151 2 bbl (80-83, GM 2.5L)
is what my VIN indicates. This option isn't listed in my Chilton's manual, even though the Wikipedia article says it was the standard option.
Of course, I've also determined that my engine in a 1978 232 with the 1bbl Carter YF carb. Someone in the past must have been tired of the four cylinder, or it failed. The transmission was probably changed to the T150 three speed at the same time. The 232 and the 3-speed were standard options from '78 and they probably came together.