1842-1845 Rocky Moutians by Captian Fremont

This was with some of my granmothers old things. She was an antique dealer / collector. I thaought the map is awsome for itself and has value. But then as I foldedit again I noticed the note on the outside. Now I know that aprox 10-15 years years prior Jedidiah Smith and Samuel Parkman prepared a map. This map was lost, but data was tranferred 15 years later onto Fredmonts map. Now the letter clearly says To Mr. and Mrs Smith. I dont know who's signature. But Jedidiah was killed in 1831 by the Comanche Indians. And the note is dated Julty 1, 1866 Washington DC. We are looking to see the value.