Lou,
Like I suggested earlier, most of the U.S. items I've had or seen that were devoid of stamped markings where there should have been some, were items that left the manufacturing plant as souvenirs or samples before being finished. That's certainly not a 100% rule, but what I've normally observed. I have a 60mm white phosphorus mortar shell that also has no roll-stamp markings. When I asked the guy I bought it from, he mentioned that his father was a War Department inspector who worked in the plant where they were made.
Regarding your 60mm round, it really isn't a practice round but an inert loaded dummy round, and intended for different purposes. They did make blue painted practice rounds during the war with small black powder spotter charges and live propellent cartridges. Your round is a unique and legitimate variation as-is and I would strongly recommend against repainting it.
I've always felt repainting ordnance is a sort of personal thing. I know collectors who repaint EVERYTHING so their collection looks very fresh and uniform. This, somtimes, in spite of finding a scarce round in 90% original paint and markings, a real treasure to me but not quite perfect enough for them. And I know collectors who never repaint ANYTHING, preferring to leave each new item exactly as found to preserve its history and the path it took to get to its current condition.
I'm somewhere in between. I'd never repaint a 90% round but if something turns up that's a 30% or 40% condition piece, I'll consider a careful repaint and restoration of the markings if I feel a faithful job can be done and the color can be matched closely. Each item should be considered on a case by case basis, IMHO.
In your case, consider that there are a LOT more inert HE 60mm shells out there than original black and white inert loaded specimens. I have a nice example like yours in my collection now but could probably find a spare HE to trade for another before seeing it painted. Just my free internet opinion though. When the dust settles, it's still your round and you can do what you please with it, especially for the bargain price it was acquired for.
