Adult supervision has arrived! Seriously, I asked the factory a few years ago, and, indeed, anything from M60-4 on is okay with Plus P. In fact, my three-inch M60 is a M60-4, the full-lugged bbl. and Target sights. Great little trail gun if no large animals are a threat.
I also have an M60-7. It shoots like a target gun with Speer Gold Dot 125 grain bullets, which are very hot for a .38, but they're a tight fit in the chambers.
I think the M60's are one of the best, most practical carry guns ever, when one doesn't need more power.
Lone Star