he tools of design were primitive. Most starting models didn't have the money to create model composites, so they began with a basic 8x10, B&W print with a solid white outline. The model's stats were added to the back in plain text.
I remember those days, printing 50 to 100 copies of the identical boring headshot. So many times. These 8x10 photos were also pretty costly, and this stood in the way of the model passing them out. Models typically sent them to companies who were likely to offer them a paying gig, or to talent agents who were likely to hire them. Models probably missed out on a handful of gigs because they couldn't afford to give out headshots to just anyone.
Over time, a model would grow to be more successful and net more money. This would let the model to make a one-color composite card made by an efficient printer. Only the top models in New York City could pay for full color. Offset printing requires a lot of investment up front, but the price became reasonable if a print run of hundreds or thousands of composite cards was put to print. Now, a model had thousands of cards on hand - and the model could definitely afford to send a card to anyone who might be remotely interested in hiring the model. The zed cards were even cheap enough to put in the mail to mail to casting agents around the country, extending a model's scope.
The model comp cards of yesteryear were a specific way due to the tech and investments involved with printing. This determined one photograph on the front and a set of photos, every one a quarter of the space, on the rear of the card. A spot was also marked off on the back of the zed card to print stats for the model and a phone number.
Printing techniques wouldn't allow the pictures on the rear of the comp card from touching at all, and you could not make use of any spiffy colors or designs. All sed cards were therefore done on a plain background, with solid white outlines. These outlines also allowed the printing press to hold the comp card as it traveled through the printing process. Printers couldn't bleed to the edge, the way today's cards and layouts do. Although printing techniques has come quite a ways, the zed cards we print these days are still derived pretty solidly on this original design, which came out necessity.