Light fitting wires

[Fleeting]

OP how far up does the right side contact in the lampholder protrude?
The picture is not clear but everything indicates it is not polarity biased.

 

[Fleeting]

This is an EN 60238 lampholder and the arrangement of the contacts and the construction of the lampholder make in non polarity biased, yours is like this I believe but a clearer picture at an angle may help.

 

[Murdoch]

But you should make the one at the bottom the live