They look like zerk fittings that have rusted up a bit.
XMHF 20pcs M10 Straight Hydraulic Grease Fitting Assortment Kit Zerk Fitting Nipple Nozzle Fitting 10mm x 1mm Male Thread https://www.amazon.com/dp/B086LJPCBK/ref=cm_sw_r_cp_api_i_tIGMFb8MCYAGZ?_encoding=UTF8&psc=1