[Note 545.1] George III reigned 1760-1820.

[Note 545.2] Stevenson could be Robert (1772-1850) but is probably his father George (1781-1848) the engineer sometimes called "Father of the Railways"

[Note 545.3] This is probably an allusion to the Night Poaching Act of 1828 by which poachers were sentences to transportation across the seas. and Game Act 1831, established a close season when birds could not be taken legally.[i] The poet is romanticising the past when he says that 'A poor man could shoot a hare, Or bird upon the wing. And not be shot by a gamekeepers'. The Black Act of 1723 included clauses intended to deter poaching by threat of transportation or even death. [ii] and was in force when George III was king.

References:

[i] https://en.wikipedia.org/wiki/Game_law
[ii] https://en.wikipedia.org/wiki/Black_Act_1723

Glossary

Garrotters: Street robbers who stole from people in the street using actual or threatened violence. Garrotte robbery was initially used to describe a particular form of violence used but Garrotter came to be applied to any street robber. [i] The first newspaper references to Garrotters appeared in 1856 [ii] and the Norfolk News of 27th December 1856 printed a description of an anti-garrotting device. There was a second garrotting panic in 1862 which resulted in the Garrotters Act of 1863 [iii]

[i] Sindall, R. The London garrotting panics of 1856 and 1862, Social History Vol. 12, No. 3 (Oct., 1987), pp. 351-359
https://www.jstor.org/stable/4285629?seq=1#page_scan_tab_contents
[ii] Morning Post (London) 11 November 1856
[iii] Sindall, ibid