what does B1672 mean on a steinway grand piano?

Question

this number is etched on the middle support under the piano

Answer 1

That is a case number.

Before the piano is completely assembled, the empty rim and brace structure is given a case number, so it's progress in the construction process can be tracked. The soundboard, the harp, the action assembly, etc., all have an identical number so that they can be matched up properly.

Once the harp and soundboard et al have been mounted, the piano then is assigned a serial number. This designates where the piano fits into the sequence of all Steinway pianos manufactured to date.