I've often wondered about it myself...
I have the single axle trailer my uncle used exclusively during his race career in the late fifties and early 60's...
He always towed with either a snappy new convertible or station wagon... they normally took both cars to each race... the station wagon was to haul the extra equipment.
This trailer has been to almost every state in the union... never a problem. It has hauled everything from Shelby Cobra, Lotus, Shelby Mustang and even a Ford GT-40 to the International Car Show in San Francisco...
No one back then had a double axle trailer and most pickups were just not as comfortable as cars in the fifties.
Fast forward to today... I use the same trailer to move my little BX-23 around and frequently am asked when I will get a real trailer or is that trailer even legal...
Well, according the vehicle code in my State, it is legal based on the date of manufacture... just like my antique cars only require the equipment at the time they were made...
My 1905 Oldsmobile is fully licensed, insured and drivable in all 50 States... no brake lights, turn signals, 4 wheel brakes, windshield, seat belts, mirrors, manufacturer load rating etc...
I have a friend that regularly used his old double AA Model A flat-bed in his business... he will put many full rolls of carpet from the mill to his warehouse... it is exempt from weight fees due to age... his dodge stretch vans can only move one roll. The Model A has only 40 hp and never runs out of torque.
He was stopped once for being overweight.... the officer called his supervisor when my friend asked how the officer figured he was over... they let him go once they found out he was 100% legal under the vehicle code...
My best advice is take it slow and easy...