In the early 60's Winchester-Western came out with the Mark 5s.They were a paper shell loaded with fiber wads and a plastic wrap around the shot.A little later on they were using a plastic shell with the same insides.Around 1968 the true AA case came out with a AA all plastic wad like can be bought today.I'm not sure how long Winchester may have used some fiber wads in their production line.I do know that the cheaper line of shells they made for the discount stores had fiber wads in them for years.I write this from memory and shooting these shells and reloading them.I could be off a year or two,here or there.I still think this burst could have been a pressure related thing with deteriated powder at the bottom of it.The other issues of rust may all had had their part in the burst also.It would be interesting to see the results of a pressure test on these shells,if any unfired shells still exist yet.Charlie.