What are true Mahler bells? He had his own bells casted somewhere, and actually dragged them around with him on the trains. I'm going to assume that he had just three bells casted, as the end of the symphony indicates for only three different pitches: high, medium, and low. Obviously, on the Zinman recording, the bells are played ad-lib. Then again, they're ad-lib on ALL of the Bernstein recordings of M2, as well as some other other ones too. They're usually played on the tubular chimes - those stupid sounding holy doorbells - so it's not so obvious when they do play ad-lib. When Mahler performed the "Resurrection" at Basel Cathedral, he had them sound the bells in the church belfry. Sets of orchestral bells are usually mounted on rods of some kind, then struck near the edge with some kind of hard mallet. I suspect that those are the type of bells that you hear at the end of Zinman's M2 (and they make their reappearance at the start of his M3/5: the "bim-bam" movement). Some orchestras actually have smallish bells that can be tipped - with a klapper inside - just like the bigger bells hung in a belfry. Obviously, you can't bring belfry size bells down to the concert stage. So, no matter what you do, it's usually some kind of compromise. It would be interesting to know if Mahler's own bells still exist somewhere. They were probably melted down for some stupid war effort.
I think that the most interesting bell compromise is on the Bertini recording. He has one set of bells - onstage - that play the part as written (alternating salvos between three bells and two tam-tams). Then he has another, bigger set of bells bonging away in the background. Since the symphony hall in Cologne is very close to Cologne Cathedral, I've sometimes wondered if he just got them to bong their bells away. That sort of cooperation happens from time to time.