Olivetti typewriters used to have keys like that. (https://www.massmadesoul.com/olivetti-praxis-48 https://www.massmadesoul.com/olivetti-praxis-48) Maybe using

Olivetti typewriters used to have keys like that. (https://www.massmadesoul.com/olivetti-praxis-48 https://www.massmadesoul.com/olivetti-praxis-48) Maybe using those as a point of reference would help in your search.