A predecessor of the AS/400, the System/36 PC was available in very similar chassis to the IBM AT. It used an IBM 5150 ( aka PC ) as the console with a special interface card.
Chronologically, it was actually S/32 => S/34 => S/38 => S/36. Though S/36 was more a successor to S/34, built on more modern hardware but didn't really offer anything new in terms of OS architecture.
Go is simply the follow on to Limbo. It is the product of many years of experience supporting the construction of large systems. Most of its roots are from Bell Labs.