Trog,
The good news is its a L42A1 rifle which was sent to Donnington secure building 69/671 on the 22nd of December 1971 out of a batch of 43. Hope this helps you with part of its history. The bolt may have been a simple cockup, somewhere by several bolts laying around and the wrong one picked up. Might be worth asking the seller if he has any other Lee's and check the bolts for you!!!
All the best
Gil