I have to guess on the origin of the print you're referring to. An inverted delta on a print (possibly from Ford Motor Company and associated organizations) means that this signifies a special characteristic.
From Ford:
Inverted Delta parts
For parts designated as inverted delta (parts with Critical Characteristics), the organization shall prevent the shipment of non-conforming product to Ford.
Note: this is typically accomplished by using error and/or mistake proofing techniques integrated into the organization's manufacturing and material handling processes.
Look
here (.pdf file) for more information.