Got this info about Holz trailers from a local 4 Star dealer:

This trailer is all aluminum, including the floor. The trailer weighs approximately 8200 lb. The measurements are: 7 X 23 X 7. The Holz trailers were made in Linden, Washington.

These trailers were found mostly on the West Coast and are no longer being made. Mr. Holz made the trailers by hand in a small shop...excellent trailers pursuant to structural integrity and quality. they are nearly indestructible. If you find one, get it.

He then offers the following tips:

...people do not comprehend the single axle Holz. He added an air ride to the single axle which is a good addition. There is a small compressor underneath the gooseneck that keeps the air ride working. Some people allowed the air to lessen and the trailer would settle onto the tires. They just did not understand how to work things. The single axle is a good item and the dual wheels/single axle can handle any load you can put on the trailer. You may have trouble finding parts for the trailer you have found. Mr. Holz began making tandem axle trailers later. If the trailer you found is a single axle and has been mistreated, be sure to check the air ride and the axle for damage. People have a tendency to bend the single axle type trailers by running over things...and they did not take care of the air ride system.

So, you answered your own question?

Other makers had similar axle configurations and you hardly ever find those still in service.  There might be a reason.  Why would you buy a trailer you couldn't find parts for?  What happens when it needs to be fixed, there would be no manufacturer to call for guidance.

Still curious about them. Especially since the dealer (highly respected around here) said, ",,,they are nearly indestructible. If you find one, get it."

There are tons of still great trailers on the road that were built by companies that are no longer in business and people manage to find/fabricate parts for them.

A really nicely restored 1974 Miley 2-horse in-line sold on E-bay Sunday, and I am trying (not enough time) to restore an old McQuerry 2-horse in-line, Both of these companies were in Ft. Worth and went out of business at least 15 years ago. But finding parts for them is not the problem - just time and money.