I’m leaning toward a cap and plug set for hoses of some sort as well. Hydraulic is the first thing to come to mind.
CO-Z 64 Piece JIC 37° Flare Cap and Plug Kit Hose Tube and Pipe Fitting Set Hydraulic Cap and Plug Kit Galvanized 45# Steel with Precision Threading Dash Sizes -04 -06 -08 -10 -12 -16 https://www.amazon.com/dp/B087TVQH99/ref=cm_sw_r_cp_api_glc_fabc_.g74FbF3VZQP6?_encoding=UTF8&psc=1