How do these state park parking receipts work?

Factual Questions
1

We have to pay $2.00 to park in the State Park down the road. They have envelopes you put the money into and drop into a locked box. The envelopes have tags you tear off , punch out a square for the day of the week, and hang on the rear view mirror, and envelope and tag both have the same serial number printed on them.

I might have guessed they would envorce this requirement by checking the contents of the locked box to see that every parked car has a corresponding filled envelope, based on the serial numbers.

But - and here’s the mystery - the tag says HANG WITH THIS SIDE FACING WINDSHIELD, and it says this on the side opposite the serial number. So when you hang the tag, the serial number is hidden against the back of your mirror.

What am I missing? How does this system work? Why is it so important the serial number be hidden?

BTW I’m happy to pay the $2.00 and sometimes pay with a $5 instead, just to help the system. I’m just curious.

2

It sounds it’s mainly a honor system. The serial number may be more like a receipt.

3

Those envelopes/tags are also used in situations where you pay the fee to an attendant at a booth. In that case, the attendant will mark the expiration date of the pass on the back of the part you hang in your windshield, so that as you drive in and out the date can be seen at a glance. That’s why it says to hang with that side facing out.