Prime Computer Inc. had transparent remote disk access across PrimeNet in 1980. You could not distinguish being on a local disk vs remote disk, other than performance. PrimeNet ran over token ring (they also had fiber optic token ring running at Ford's main Dearborn, MI campus), full-duplex synchronous, and half-duplex dial-up synchronous network connections.
Prime had remote procedure calls. Their MIDAS product (index sequential files, basically key-value store with multiple indexes) allowed multiple nodes to access the same file by allocating a "slave" on the node hosting the file, forwarding all calls over the network, and returning call results. OS file access worked the same way, using the same slave for all remote procedure calls. Source code designed for local file access didn't require any changes to be used for remote files, just like NFS.
I'd say Sun had a pretty good example to go by for NFS. In fact, if you want to see this in action, I wrote a Prime minicomputer emulator and have remote disks available between 5 different revs of their operating system, spanning a 10-year period.
Telnet em.prirun.com 8001 and enjoy!