Oil plug for a draper oil drainer. The more expensive version of this try have 2 plugs so the air can be exhausted through another whole. I drilled a hole and printed a new plug. Rubber washers can be bought on amazon:

https://www.amazon.co.uk/gp/product/B07QGCL1WP/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1