Forgotten model indeed. What most people forget is that this was shoe released in the same seasons as the Air 180 and was actually the official 1991 Air Max model, i.e. Air Max 91. The magazine ads for this shoe were shown on two pages, usually side by side. On the left side an infrared Air Max 90 was placed above the words "Air Max". On the right was a Black/Persian Violet Air Max 91 above the words "Air Even More Max".
The black and purple color was quite controversial where I came, from but the shoe was very popular. It was kept in production for years but they began selling it as the Classic BW in order to differentiate it from the current Air Max models of the following years, e.g. Air Max 93, 94, 95, the two 96 models, and Air Max 97.
The Air Max 180 was actually just called the Air 180 back in 1991 when it was released. During the lead up to the 2006 release of the Air Max 360, Nike developed the history of air campaign, with lots of advertising, posters, etc. They re-issued lots of old Air Maxes in original colorways. The campaign focused on the history of the development of the Air Max units though so several Air Max models were overlooked as they re-used air units from previous models. The Air Max 91 had a slightly larger air unit than the Air Max 90 but the Air 180 was slightly larger again so that was shown as the 4th evolution of the Max Air technology. The shoe name was retconned into the Air Max line and (http://en.wikipedia.org/wiki/Retroactive_continuity) and the name changed to Air Max 180.
They also ignored the Air Max 94 which used the same 5th gen air unit as the Air Max 93. This model is largely forgotten, though it is almost the same as the Air Max Burst retro that was popular around 2000.
The two Air Max 96 models basically used the same air units as the Air Max 95 so were also ignored in the campaign. They are generally held as the ugly ducklings of the Air Max line, along with the tubular air model released around 2000.
The Air Max 97 model was the next evolution presented as part of the series. The tubular air was a bit of a flop so was ignored.
It's a shame that Nike don't really recognize this shoe with it's original name anymore. Hope you enjoyed this history and enjoy the new Air Max 91 retros. :)