The Semi Beavertail forearm is the Semi Bevaertail forearm whether installed on a "B", "AB" or a "V". The only differnce is the inletting for the different barrel profiles. There is no "V" semi beavertail in terms of exterior dementions compared to a "B" or "AB" The only difference at any point in time is the barrel inletting for the different barrels. It's one of the reasons the "AB" sort of looks awkward or off balance.
The Semi Beavertail forearm changed exteriorly over time. The original version was quite rounded and the tip was very round. By the time yours was made the bottom was flatter and the tip was a bit squarer. By 1978 it was quite flat on the bottom and the tiip was quite square. The only change since then is the change in the angle of the front of the reciever in 1984.
Unless you are very familar with the variations or just happen to have all three on hand to compare the easiest way to tell would be to look on the inside of the forearm. There should be a date stamped in the inside bottom of the barrrel channel. This is the day the first coat of finish was applied to the forearm. While not definitive it is probably the best indicator you can find to show when the forearm was installed.
Check the Clayton book, you are looking for the "second style".
Just saw this, take a look at GunBroker auction 203297385 pictures #2 and #4 do a very good job of showing the rounded tip on the forearm and most of the other features. Your's should look like this one to be correct for age.
Ross