The 2005 Gibbs stamp, with James Maxwell's thermodynamic surface, derived from Gibbs's work, shown in the background. 
“We started getting material together. We got through the first cut, and they wanted suggestions for a design to go along with Gibbs’ picture. I argued that the design had to be related to Gibbs' famous threedimensional energyentropyvolume surface, and we finally settled on the map of the USV surface that I had dug out of the tombs at the Berkeley Library from Maxwell's 1875 Treatise on Heat. USPS accepted the idea and sent a draft design to me about a year and a half ago for comments.”
The "famous" threedimensional energyentropyvolume surface, referred to here by Jolls is likely the “statue of water” plaster model (of which three were made, two remaining in Cambridge) that Maxwell had sent to Gibbs in New Haven in circa 1873. [13] JoIIs also convinced the USPS to rename Gibbs as a thermodynamicist, rather than its original title of chemist, upon his recommendation.

Fig 26d. "Thermodynamic Surface" in James Maxwell's 1875 book Theory of Heat, visualized more fully in Maxwell's thermodynamic surface. 
dU = TdS – PdV
dε = tdη  pdv

Letter to editor: "Stamp of Authenticity", April 08, 2006 Mechanical Engineering, by Zollti Spakovszky. 
“I couldn’t take my eyes off the Gibbs stamp. I noticed that there were some dark spots on his collar. A closer look under the microscope revealed a very pleasant discovery: the Gibbs equation is written on his collar. It is in the original form Gibbs reported on the second page of his paper, “Graphical Methods in the Thermodynamics of Fluids,” published in the Transactions of the Connecticut Academy, 1873.”
In 2009 commentary, Spakovszky affirms that finding the equation hidden on the collar was a “delightful experience”. [9]