Hi
@dogdog1
So the light you show above are for a Series landrover not a Defender. There are some slight differences and I guess this is what you are refering to as them being 1 piece.
Like below ....
View attachment 256260
Both of these are single fillament bulb holders, the dual fillament have three wires/connections. Red, Green and Black. Black is earth.
To test the bulb and holder you need a 12V battery (I use an old motorcycle battery) and some wire with small clips - it's a really usefull addition to finding electrical problems. I would reccomend an inline fuse on the positive side too especially when searching for 'problems'.
Anyway back to your question.
The orange indicator bulb holder (above) won't fit into the hole that is designed for the lower (sidelight) holder.
The top indicator holder screws onto the wing/panel which also forms a part of the earth and then the orange/clear/red lens screws onto the base.
The lower holder screws into the wing/rear panel and is attached/glued to the lens.
Here is a pic of my 'Fokker', you can see the different sized holes as the Fokker has a Series wing and a Defender wing ...
View attachment 256267