They are brass so filing is no problem. Some of the ones I use are uncored which means metal has to be removed to insert a bulb or to light number boards. You can make lenses using clear plastic.
This headlight had to be opened up for a lamp.
Pete
Neat. I did find a couple in the Lionel parts list. I wonder if being a toy train company, if they use the same pats for both S & O. I also wrote to their parts help people a couple of days ago -- nothing yet.