I never thought about this, but since there are at least two of each sticker on an uncut sheet, the patterns for Ludlows could potentially be different on the backs of the same sticker. Has anyone delved into collecting these variations? Those variations could probably be used to validate fake Ludlow-backed stickers too.
Vintage Ludlow sheets varied from sheet to sheet (sheets were cut from a larger roll) and weren't always fed into the press in the same direction.
1st Series sheets had 2 slits consisting of 7 rows of 11 stickers (77 stickers per slit, 154 per full sheet). 2nd Series and later sheets had 2 slits consisting on 12 rows of 11 sticker (132 stickers per slit, 264 per full sheet). Starting with the 2nd series they denoted Slit A (*) and Slit B (**) with stars in the copyright line. There were 2 types: red/permanent and black/removable. I'm not sure if anybody has truly been able to recreate the pattern printed on the back of the larger sheets, from the known stickers for either type.
Modern Ludlows (OLDS, etc pre-POD) were printed onto the back of the sheet and were uniform from sheet to sheet with some of the series like OLDS3, having 2 intentionally slightly different Ludlow sheets, so each sticker is available in 2 back variations.
I have not bought any of the POD issues, but judging from the ones shown on ebay, the Ludlow back is identical on each sticker and in the case of at least the 2020s they erroneously created red/removable ludlows.