Reminds me of default passwords on wifi routers a decade ago - ATT especially had a very identifiable SSID format (ATT###), and a default 10-digit password. That leaves you with (9,999,999,999 + 1 =) 10 billion[1] passwords possible, which even at that time only took a couple hours to test all of them. That SSID pattern also left you with only 1,000 possible SSIDs, so a rainbow table was definitely reasonable.
[1] - though now that I think about it, that might not properly cover the case of leading zeroes in the password, so the total number of possible passwords might be larger than 10B; that's assuming a naïve password list generated just from numbers, not from treating the digits as characters, so I need to reason about this a bit more...