User blog comment:K6ka/EA has announced a new spinoff: Grand Theft Plumbob!/@comment-4632631-20150416130357/@comment-1997588-20150416131035

It has always been debatable. Both spellings have been used inconsistently throughout the series, so both of them are actually acceptable. Most people now commonly use "Plumbob" in TS3-4 eras. "PlumbBob" was more commonly used back in TS1-2 eras, but these weren't always the case.