G= 1986 model year
10E = month/week from scheduled start of build. In this case, 5th week of October, 1985 for scheduled build week. Somewhere between 10/28-10/31. However, this didn't always corrolate exactly. If any delays due to parts or other issues it might push it into the next week, but that normally didn't happen. Build sheet or paperwork from GM heritage center can give you the exact start of build.
GR47= 2 door Supreme
P= Pontiac build plant
042588= Fisher body number for Fisher plant use. Has nothing to do with the VIN.
12L/12U= Lower and upper body color, obviously in Lacquer. In this case, 12= Silver Metallic
79B= Claret cloth interior
AM6= Bench seat 60/40 with center arm rest.