Nice photo :).

Is A23 B354 the lowest serial number of 50 new A23?

Thanks! So far - yes, B354 is the lowest. Not all vehicles are delivered yet so there is still a possibility to "discover" new lowest number ;)

I see.

If A23 B354 is the lowest number then A23 B403 will be the highest.

Possibly. But still, please keep in mind that not all vehicles might be delivered in the order they were built and there might be gaps: few vehicles of other customers might be assembled alongside.