For all "1992-1997 Opel Frontera" and "1992-1998 Opel Frontera Sport", it's Mk1.
https://www.imcdb.org/vehicles_make-Vauxhall_model-Frontera.html
https://www.imcdb.org/vehicles_make-Vauxhall_model-Frontera+Sport.html

For all "1998-2001 Opel Frontera" and "1999-2003 Opel Frontera Sport", it's Mk2.
https://www.imcdb.org/vehicles_make-Vauxhall_model-Frontera.html
https://www.imcdb.org/vehicles_make-Vauxhall_model-Frontera+Sport.html