One sees '1831' on a lot of Welsh labor flags and banners. This is in reference to the British government murder of Welsh labour martyr, trade unionist, and coal miner Dic Penderyn. See the Dic Penderyn Society site here.
Here is a lovely Martyn Joseph song concerning Dic Penderyn:
I don't know why the flag in this video is shown as blue, every description of the flags at the Merthyr rising I have read describes them as red. Indeed, Wiki's page on the Merthyr Rising states that this was "the first time in the world the red flag of revolution was flown" - though it unfortunately gives no citation for that. The page on the red flag at Wiki seems to contradict that, as it describes the red flag as a Jacobin symbol of French anti-royalist martyrs. I suppose Merthyr might have been the first use of a red flag in a specifically labour self conscious as organized labour oriented revolt against bourgeois power. I'll have to investigate this more seriously as time allows. I like the idea of the red flag having origins connected to the Welsh people, Welsh red, and the red Welsh dragon. I think Paul Robeson would have greatly appreciated that.
There is this brief video which mentions the use of the red flag (washed in the blood of a calf):
Lastly, this song commemorating the Merthyr uprising. You can't really understand most of the words, but the beat is sort of there and it seems like they are having fun: