Denver steak is the traditional name in United States. Elsewhere, butchers sell the same piece — or a close cousin — under a different name.
Both come from the chuck primal and are prized as more affordable alternatives to ribeye; aguja norteña is the chuck eye, Denver is the under-blade.
Same anatomical zone — Denver steak is the modern US butcher cut from the chuck-roll inner muscle, diezmillo is the broader Mexican-tradition cut from the same general area.