Hello Leonard and welcome to our site.
It does look quite a decent twin you have there. A 1979 model is most likely to be a B4 although it could be a B3. However after saying that the front end looks to be B1 or B2 as it has a leading caliper which was trailing on the B3 and B4. Maybe the spare frame has been a donor of parts and is an earlier model. You can look in the Model Identification section for frame numbers or alternatively give us them to us and we can clarify this for you.
Enjoy your new found 2 wheeled world and keep us posted of your antics. If you need any help please shout up.