Somewhere along the line some people got the idea that the 2105 put out 105 watts and the 2100 only 100. Don't know where those extra 5 came from, maybe the glass? :-)
They were both rated at 100 wpc but actually put out around 150. The first 3 generations of SS Mac used a 5 at the end of the model # to indicate it had glass and meters. I think it was nice that they offered the same amp at a much lower cost if you didn't want the fancy trimmings. Enjoy your classic.