Transcription from Arlington Vital Records:

[DEATH] "HALL: Thomas Jr., [Feb.] 25, 1770, a. 39. P.R."

The carved design on this gravestone is the right period to possibly be this person, but it is difficult to say for sure without being able to see the full stone.

There is another candidate from Arlington Vital Records' transcriptions from slightly later:

[DEATH] "HALL: Thomas, Sept. 8, 1804, in 44th y. G.R."

This person is the right age to be Thomas Jr.'s son who was recorded in the Arlington VRs (Transcription: [BIRTH] "HALL: Thomas, s. Thomas Jr., July 4, 1760. P.R."). EDITED TO ADD: In 2013 I photographed the gravestone of this other Thomas Hall and added a memorial to him.

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[Below added November 2014]

Fellow local FindAGrave member Denise (who also isn't related to Thomas) believes that the following transcribed vital records also pertain to this Thomas Hall (it remains too bad that the stone is too sunken to show even his age and death date):

"Thomas was the husband of Rebecca Cutter. They wed in Medford, MA, on Nov. 11, 1756. He was the son of Thomas Hall*. He was baptized in Menotomy (Arlington), MA, on Aug. 6, 1733 at the First Congregational Unitarian Church.

Children w/Rebecca
(listed in both Arlington & Cambridge Vrs births)
Thomas, July 4, 1760 - Sep. 8, 1804
Ebenezer, Dec. 24, 1762 -

Children (b. Arlington)
unnamed, Oct. 17, 1764 - Oct. 17, 1764
Gershom, May 28, 1767 -

_____________
-Cambridge Vital Records, births, pg 319
Hall, Thomas, s. of Thomas, bp. at Menotomy, Aug. 6, 1733. CR1

-Cambridge Vital Records, marriages, pg 179
Hall, Thomas (Jr. int.) and Rebecca Cutter of Medford, in Medford, Nov. 11, 1756.
-Medford VRs, marriages, pg 236
Hall, Thomas of Cambridge and Rebecca Cutter, Nov. 11, 1756.

*He is definitely the son of Thomas, as all the records state, 'son of Thomas'. I believe his mother was Patience Allen. (Thomas Hall and Patience Allen Wed in Cambridge, MA, July 10, 1729.)"