Pictures sometimes help as small details can help narrow it down. A VIN
or serial number may provide clues. Also, check to see it there is anything indicating place of manufacture. Early units (which would include yours if it indeed dates to the 70's) were made in Iowa and later units in California.
You can use the "Manufacturers" tab at the top to navigate to the Burro
page and look through old threads for clues. Unfortunately it is not always possible to narrow it down to a specific year, as molded trailers don't tend to change much from one year to the next.
Like Glenn, I wondering about title
or other paperwork to indicate date of manufacture? Some states don't title
trailers like these, and even those that do can have incorrect information after so many years.