This blotter measures 6" X 3½" overall. Blotters were a popular form of advertising before ballpoint pens took over. The fountain pens in use back then would sometimes leave excess ink and a blotter card was a handy way to remove it. The blue felt backing on the back of the card readily absorbed the ink, note that this blotter was only used a little. 
Holt Machinery Co. is the largest Caterpillar dealer in Texas and hosted the 2018 ACMOC Show.
I think the style of the decals on the fuel tank of the D6 should help determine the age.
Thanks,
Craig
             

Looks like an 8U.

The original appearance of the D6N was on the 4R, 5R D6. I'm thinking the blotter picture is of a 5R.

Thanks OM, I thought the D6 was an older model. The way old phone numbers was another clue.
Craig