Hello and welcome Paio, your Milners looks like a List 3 Strong Holdfast.
The original keys were a bright steel, not plated, and usually had a distinctive bow shape with the oval stretched slightly where it joins the key stem, like the key blank pictured below.
They also used pipe keys where a hole up the end of the key fitted over a fixed drill-pin in the lock, not sure which yours is.
Not sure on your handle, if it’s original it’s certainly missing the rose (round back-plate), but many also had brass knob styles handles instead of the lever.
You’ll need to find the serial number to narrow the date down, look on the top of the door, on the edge and to the right near the top hinge carriage.
Think the bodies were often black but they used green or maroon for doors and insides were nearly always a beige/buff brown on the ones I saw.
The powder-proof lock isn’t to any recognisable security level but they are amazingly pick resistant, solid and incredibly durable. I still cannot recall ever seeing a broken lever spring in a Milners lock and I literally saw hundreds of them!
