A while back, a turbo targa Holbert built for a customer was for sale. Here is a link to the ad:
http://davidsclassiccars.com/porsche/45466-78-930-turbo-targa-alan-holbert-editon.html
In the ad write up for the Hobert built turbo targa (built on a 911 chassis), it mentions Porsche's interest. As a result, according to the ad, Porsche ended buildling 2 turbo tagas in 1978. The ad also notes these 2 Porsche built turbo tarags were in an Excellence (#50 December 1994) artcle. I'm not saying this is or is not one of the 2, but sharing the information.