Temperature!
A hint: at least among older devices, or from some manufacturers (National for instance), the first digit describes temperature rating. LM117/217/317 for example, the regulator for mil/industrial/commercial temperature ratings. Most parts these days are already made to run up to 125 or 150°C so it's not a big deal, but they were a bit more limited back then.
SG2524 happens to be rated 85°C; 3524, 70°C. Unitrode also used this scheme, with UC1842 rated 125°C; 2842, 85; 3842, 70.
The SG prefix was SGS Thompson, bought by, various I think? ST Micro (well, ST is "SGS Thompson" renamed..!) and on semi at least. Several second-sources have made them over the years, and TI remains among them today.
It doesn't always hold, of course; LM2903 is basically a LM193, in plastic package, with moderately degraded characteristics. After they used up the first few hundred numbers, National I think got a bit more free-form with their numbering scheme.
Tim