The B8 code is short for B80 which is Roof Drip Scalp and Windshield Pillar Molding. Here is a thread from a few years back on decoding a similar car:
Hello to all, just joined the forum. Looking to decode the cowl tag on my 69 GS California. I bought this car about a year ago. It appears to be original except the color. It's red with a white vinyl top. The engine matches the VIN. It has a 2.73 open rear and all the info I can find says it...
www.teambuick.com
Here is a list of the RPO codes showing B80:
The BW prefix for the Wilmington Fisher Body plant is shown in the Buick Master Parts Book of 1972. It's on the '64 model page which is the last one in that book to show the body plant prefixes. It would have still been valid in '69.
The Y code for Wilmington assembled cars shows up on the VIN plate which was produced by Buick Motor Div. Back then, the Car Division and Fisher Body had a lot of independence - almost like two separate companies. On some cars the body was not built at the same site as where the car was assembled, so both plant codes are necessary.