Farley and Manuel,
The cancel is Sz 1749 and is one of the most common used on the 1861 stamps from Veracruz. I have attached a picture of two examples, with one showing part of the inner circle and another without this. The inner circle apparently just does not show well. Actually, judging from the prints the one that shows part of the ring was used later than the one that I have pictured on the right that appears to be an early print. The stamp you have pictured also appears to be an early print.
Regarding the use of the US postmark 'Old Stamps", I think that what may have happened is that the Mexican stamp was applied to the cover at the VCZ post office. A un real was required for delivery to the ship that may have taken the cover to a US port, such as New Orleans. A US stamp was applied somewhere along this trip, and likely in the US. During the time when the new issues had been printed and the old issues were still in the hands of many people, especially businesses, people were still using them without realizing they were no longer valid. So, when the letter arrived at say New York, it was stamped with the 'Old Stamps' regardless of the fact that the Mexican stamp was still valid. The reason that it is possible to have a lone Un Real on the cover is that in 1861 almost all covers going overseas were sent due or only paid ship carrier fees from the port.
Just my take on this interesting stamp. Too bad this was ever removed from the cover. It would have been worth tons of money.
