All in all they were well made rifles, and in my opinion just as good, if not better than the much vaunted pre-64 Model 70 Winchester.
If the one you are looking at has a Mauser type extractor on the bolt it is probably an Imperial (Also sold as a Royal in different markets) If it has a ported barrel and looks to have a very trim sculpted receiver and stock it is a Featherweight.
The bluing on the barrels usually wore thin long before the bluing on the receivers. The checkering on the stocks was hit and miss. Some were very good while others were not so great.
No one that I know makes an aftermarket stock or trigger for the BSA rifles so you will have to live with what it is wearing when you buy it. The good news is that their triggers, while very complicated are quite good.