B311 was simply the facelift of the B310. In USA, and probably in Australia, the cars were not marketed or ID'd this way. It seems Japan had more a fascination with ever-changing model numbers.

The switch from round headlights to square headlights came mid-1980 if I recall correctly. Early 1980s had round, later 1980s had square. This is of course confused by the concept of "Model Year".

The slope-back wagon was known as the "Sunny California" in Japan, and was the only wagon we got in USA from 1979-1982. We didn't get any squared-off-back wagons.

That looks like square light sunny gear. Arnt the slope back sunnys all round lights (pre 1980)?

If I remember rightly, there were two different B310 station wagon bodies. The one we got had a steeply raked rear window, & in some other parts of the world, they got the better looking, more squared off version. More bloody practical too.

I wonder if this is the mysterious B311 since everything else on the car was B310 & the change of code would identify that particular body.
Just a guess.

EDIT
Just realised, seller is in Brisbane, might be worth an ask.

I noticed on ebay that a company is selling stuff for a Nissan B311, do they exist??

http://cgi.ebay.com.au/NISSAN-DATSUN-SUNNY-B311-FRONT-FENDER-GUARD_W0QQitemZ350023292666QQihZ022QQcategoryZ6763QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

http://cgi.ebay.com.au/NISSAN-DATSUN-SUNNY-B311-GRILLE_W0QQitemZ120219759200QQihZ002QQcategoryZ6763QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Looks like B310 stuff to me.