That's interesting - I like oddball stuff. Maybe someone else will chime in but I kinda remember something like this where a color was available from another division & was special ordered (SCO or Special Car Order in Buick world). The correct color material/top may be available from SMS (https://smsautofabrics.com/).
I've seen this anomaly before. I'm guessing late '70 cars had a new '71 color available if the salesman new how to do it.
