I find a couple of references to this particular 'Act of Oblivion' or General Amnesty of 1686. The first appears in the Proceedings of the Somersetshire Archaeological and Natural History Society, By Somersetshire Archaeological and Natural History Society. (emphasis mine)
Not far from the Church is the fortified entrance to the town it
consists of a stone archway with a Chapel above which is generally
supposed to have been a wayside Chapel It is called Hanging Chapel
from the circumstance of three persons having been hanged there for
being engaged in Monmouth's rebellion by Judge Jeffreys about 1685
their names were Humphrey Pierce Nicholas Venting and John Sellwood
James Hurd of Langport was one of the persons excepted from the
general amnesty or act of oblivion issued by James II 10th March 1686
Note this entry gives us a date and that the source of this 'act' is the king and not parliament. Another source is from a footnote in Notes & Queries for Somerset and Dorset, Volume 2,edited by Hugh Norris, Charles Herbert Mayo, Frederic William Weaver. This agrees on the date previously mentioned, and provides an original source for their information:
A general amnesty or act of oblivion (with however, many exemptions
amongst which Mr Prideaux's name does not appear) was proclaimed by
the King on March 10th 1685-6 (British Museum, Anglia 1684-1792 iii.
So this original information may be available in more detail in a work in the British Museum archives.
Another work, The Life, Progresses, and Rebellion of James, Duke of Monmouth..., Volume 2, By George Roberts (1844) also indicated the same source in the British Museum.
James II in his proclamation 10th March 1686 declares he is persuaded
that many who joined themselves in that rebellion being poor labourers
and handicraftsmen were drawn and seduced thereinto by the subtle and
crafty insinuations of some ill disposed persons of greater note and
quality than themselves and not from their own evil rancour of mind
and traitorous aversion to him or his government All who in a
traitorous and hostile manner landed with James Scot Duke of Monmouth
and were officers or had the name or were reputed officers and the
following individuals were EXCEPTED:
This actually goes on to provide the detailed list of those who were excepted (not provided amnesty) from this act.