According to this page.
Back in early 70s', Nissan offered this type spoiler only to NISMO authorized racing teams and shops in Japan.
I've got great respect for Jay Ataka of JDM Car Parts, but sometimes his knowledge of the Sports Options and race parts is not really up to snuff.
Two problems with his statement: 'NISMO' didn't exist by that name until 1982, when Nissan's in-house race divisions and the standalone Nissan Omori "factory" were realigned to form a 'new' division. Previously Nissan's in-house race departments had been known as 'Nissan Sports' and various other monikers, and the selling of the race option and sports option parts was spread around a bit too much. With the formation of NISMO the idea was to rationalise and centralise as much of this as possible. Nitpicking I know, but why refer to 'NISMO' in a time period when 'NISMO' was a Telex address?
Second problem is that no such spoiler can be seen in any of Nissan's 'Sports Option' or race option lists and catalogues for the S30-series Z. So if Nissan were selling it, how were they marketing it? Was it some kind of secret? No, the answer is that it's in fact an aftermarket part based on something originally made by a company other than Nissan themselves.
It's very similar to an item sold in the early 1970s by Scuderia Nissan (despite their name, nothing to do with Nissan) who were racing a few S30-series Zs and had a thriving FRP parts business. My guess is that Scuderia Nissan may have been the original authors of the design, but it's been copied and copied and copied for the best part of 45 years since then...