An old Pentax
 

Have got first refusal (sort of) on an old Pentax SMC 1:4 45~125 Zoom lens, in its original leather (?) case. It is part of a load of photo gear which was used for 'O' level photography about 15 years ago at the school where I teach.
I've got the lens at home at the moment and have given it a small cleanup as it was covered in 15 years of Art Department dust and crap. Apart from a small dent on the very front it appears to be in very good condition, and seems to take pretty good pics.
I've done a search on E-BAy etc. but cant find any info. Does anybody know this lens and what it might be worth?