There's an auction lot on Philasearch that has all three varieties (three "a", 1 each "b" and "c"). They weren't in a single scan together, but assuming they were scanned at the same time, the two scans should be close. I combined them below. Click to enlarge.
Thank you both. Using the composite image above, the most clear diagnostic for 1b appears to be the color of the 30pf. It's less saturated and more of a steel gray than blue. Visually, I'm not seeing much differences in the green of the 10pf between the sheets.
Admittedly, there's only so much one can tell from online images as far as absolute color is concerned, it's more about the comparative differences.