Hello,
I had a 1969 BS100 and that was the first year of the tube type frame. Your bike has a stamped frame so it is likely to be older than a '69. That muffler is also an older version of the style I had. The S/N guide will tell you exactly which month/year your bike was manufactured. For quite some time in the US, a new bike was frequently titled for the year it was first registered. So a leftover 1969 bike purchased new in 1970 would show up in the records as a 1970. I'm not sure when they finally stopped doing this but my '69 bike was titled as a 1970. As said previously, search this site for brochures and make note of the different frame types.