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What an interesting document. I see nothing to suggest it’s anything other than authentic. Here’s a little information that might help you in researching it.
According to the “History of Centre and Clinton Counties”, written by John Blair Linn and published in 1883, the “Lower Bald Eagle” township of 1801 divided in 1817. Part of it changed its name to “Spring”, and the part of it between the Muncy and Nittany Mountains was then erected as a new township called “Lamar”, named in honour of Maj. Marion Lamar, of the Fourth Pennsylvania Line (he was killed at the Battle of Paoli during the American Revolutionary War). When Clinton County was organised in 1839, it embraced the Lamar township plus a number of others that had previously been part of Centre County.
The number of Lamar residents in 1817 was sufficiently small that the men are listed individually by name: 135 of them plus a business in joint names, and 27 “single freemen”. In the 1st June 1830 census, at which time Lamar was still in Centre County, the population was recorded as “1567 Whites and 15 Colored” [sic].
Your document appears to have written by a notary or someone of authority who was perhaps more literate than the two men entering into the agreement and the bottom signatures may or may not be in the hand of those men. As such, it suffers from the usual problems of unreliable name spellings (as well as the difficulty of reading old fancy handwriting). Imagine the difficulty in getting someone else to write your surname if you, yourself, are illiterate and can’t spell. All they have to go on is the way you pronounce it, and you have no way to verify it’s written correctly if you can’t read. Worse still if you have a foreign surname.
I read the names at the bottom as “John Heglich” and “Philip Rishal” but in the body text the surnames appear to be written as “Haighleg” and “Rishel”. These kinds of inconsistencies are not at all unusual. From the 1817 list of resident males in Lamar, there is a “Philiip Rishel”, but no matches to the other surname, however you spell it. In some cases, occupations are given in that list, but unfortunately not for Philip Rishel and no other details. Alongside him there are also listings for Adam Rishel, John Rishel and William Rishel (again, no other details), but I would presume they are family members.
You might try to track down the census of 1830 I referred to above, if documentation has survived.
Good luck researching it further.