I have heard that many times over the years as well.NOT SAFE to shoot if it is MK3 .308. Only Enfields safe to shoot in .308 are from Ishapore.
OP, How did you determine this rifle is a 308?
Reason is, are your sure? Could be a 303, and those do look very similar. Can you post any pictures of the markings that denote the chambering of the rifle if possible?