Very odd. Delta Cruisers aren't made in 597 and I'm not sure they ever have been. Even then, that being wrong would produce the opposite problem, of a tyre too loose, not too tight. Similarly, Rigida 597 rims seem like they may be stamped 26x1¼" (don't ask) if they indeed exist, whereas ones stamped 26x1⅜" are 590 BSD.
So I don't know. I've never struggled getting them onto rims, even in narrower sizes, although I usually use levers regardless, but Deltas have been about the easiest non- MTB tyre, fittable even with free gift tyre levers that seem to be made of cheese. I hope it's something freak like a bad batch or one of those rim/tyre combinations that just doesn't work.
Don't they have the ISO size embossed on the sidewalls? I'm 90% sure my 37-590s do and if you're really desperate, I'll go out in the fog and check tonight
Can you measure the diameters and see if either are grossly out? (You'll need calipers to straddle the wheel hub and avoid tape bulge, or to do some trig.)
