That’s a post-war pre-1958 Junior. It has a batch number, not a serial, making it post-WW2, and the Brum 4 means pre-58, when the factory moved to a different post code.

The one Baz posted a pic of is pre-war (including having an actual serial number prefaced by a J). Hence why the buyer may have spent a lot on it. Though I personally think £170 for a Junior is a bit much.

I don’t much like Juniors, but £60-80 all in for a decent boxed post-war one doesn’t sound unreasonable these days.