1994 Grand Canyon Souvenir Sheet

I purchased a souvenir Sheet in 1994 at the Grand Canyon National Park office in Arizona. The Sheet states: "Grand Canyon National Park 75th Anniversary".
It has 4 ten-dollar denomination stamps in the center of the souvenir sheet, with perforations between and around the stamps. It also has a sheet number in the upper right corner. I have never been able to find this sheet in any catalogue, and local stamp dealers in Houston are mystified concerning this sheet. Does anyone have information on this sheet?

Is it a revenue sheet? Is it a souvenir sheet for day-passes? CAn anyone provide any information?

I can provide an image, but will do so only when information or interest is indicated...

Kind Regards,

Keith9

These are cinderella stamps printed by the Grand Canyon Association in 1994.
They are not US postage stamps and have no postal validity.

A sheet was recently sold on http://cgi.ebay.ca/Grand-Canyon-Ass...el_W0QQitemZ160258096805QQihZ006QQcategoryZ262QQcmdZViewItem

Thank you very much.

Keith