Maponus/Mabon at Camboglanna/Castlesteads

In Arthurian tradition, Uther Pendragon has as his servant the god Mabon son of Modron (PA GUR), while Arthur numbers Mabon among his retinue (CULHWCH AC OLWEN).  I find it interesting, therefore, that Arthur's Camlann - the Camboglanna Roman fort on Hadrian's Wall - has a Roman period dedication to Maponus: 

https://romaninscriptionsofbritain.org/inscriptions/2063

Haverfield was doubtless correct in assigning the stone to Castlesteads.  From

https://archaeologydataservice.ac.uk/archiveDS/archiveDownload?t=arch-2055-1/dissemination/pdf/Article_Level_Pdf/tcwaas/002/1913/vol13/tcwaas_002_1913_vol13_0021.pdf:

(26) It is necessary to add a similar note about an altar dedicated DEO MAPONO by various Germans, which was noticed about 1755 at Nunnery near Kirkoswald, four miles east of Plumpton Wall. This was assigned by Lysons and Bruce to Plumpton Wall (see Pegge, Gentleman's Magazine, 1755, pp. 392, 438; Lysons' Cumberland, p. clxxiii. ; LS. 793) . But it was really found near Brampton, that is, at or near the fort of Castlesteads, about or before 1688. This is attested by a letter of W Nicolson, later Bishop of Carlisle, dated June 5th, 1693, which is preserved in the Bodleian Library (MS. Ashmole 1816, fo. 466. See CIL. vii. 332, LS. 793, and Eph. Epigr. ix. p. 566).

I say this because the dedication was erected by four Germans, and we know that according to the NOTITIA DIGNITATUM the Germanic Tungri was the chief garrison of Camboglanna  (see http://lukeuedasarson.com/NDtungri.html).