Wow this is an old one haha! I barely remember that happening. In the end, I bought a set of headspace gauges myself - can't remember who I got them from but they are the disc types that you use without removing the extractor. The rifle passed with flying colours with go, no-go and field gauges. I have fired a good few more rounds through it since then with no more cracked cases and no blowing up.
I did have one more issue, when I pierced a primer on a Prvi round and got a face full of smelly gas, and the cocking piece came back and bit my thumb...but in the end, based on advice from others, I put it down to a bad round (the firing pin protrusion was within spec)
I don't really use that rifle much now and probably haven't fired it in almost 2 years.
One thing...I was wrong when I said I had a brass case crack...I thought that Bulgarian stuff was brass but it's steel. I have actually never had a brass case crack.
I'd suggest getting the rifle checked out...especially headspace. I got my own gauges because the local gunsmith couldn't find his 7.62x54R gauges. But I was glad to have him inspect the rifle and cases visually anyway...that and the headspace test gave me some piece of mind.
I have no idea about deliberate marks...try to get some pics of the marked/cracked cases and post them up.