cross member stencilled/painted markings

[JonE]

Perhaps they had lost the N and simply used a Z as it was to hand?

but do you see the font for the first and second bit as in any way different? If they are not, then its impossible they have lost it! Especially as Tony has said there is another plate identical. 

Unless AUSN and SLN were stamped at different times.

[Tony Press]

It's still back-to-front, even if it was a Z!

Tis too - curiouser and curiouser

[Nick Lettington]

The letters align and they are deeper at the top edge. Both S have a thicker top stroke. I think both AUSN and SLN must have been set together in a tool and stamped. The numbers however have clearly been individually stamped.

The backwards N may have come from a set used to raise letters from the back (such as the N of No.)

Regarding the need for SN at all, are the springs stiffer for the saloon? If the majority of cars were originally tourers, you'd only need to know when a saloon chassis was being built up.

All surmised. No facts in fact!

[Chris KC]

Yes, discarding my wilder theories I'd guess EX was for export, on a production line you need a big clear indication when something comes along that requires any different spec parts. (Does the pool of vehicles exhibiting the marks support this idea?)

Also if bodyless cars were being shipped the recipient would need to be able to clearly distinguish between any variants (saloon vs tourer, for example).