Story of how I found them, for reference:
T5631, from 2005, discusses the long-resolved issue of users being able to register a user name resembling an IP address with the last octet obscured (e.g. User:123.123.123.xxx). That was a security hole at the time, allowing masquerading as the owner of edits imported from the UseModWiki database - UseMod had a feature of displaying IP addresses in that fashion.
Once that hole was closed, one user created some user pages with titles of the same form to identify the issue. Those were subsequently renamed by the maintenance script in the run up to SUL finalization. For example, the page "User:200.191.188.xxx" was moved to User:Invalid username 480779~enwiki. I did a search for user pages matching User:Invalid username... and there are only a handful, which led to this discovery.