What's the meaning of numbers before an EPSG code, e.g. the 6.9 in EPSG:6.9:4326?

A WFS server I was using offered boundaries data in the following EPSG codes:

• EPSG:6.9:27700


• EPSG:6.9:4326

...defined in the XML giveCapabilities spec like this:

urn:ogc:def:crs:EPSG:6.9:27700
urn:ogc:def:crs:EPSG:6.9:4326

As I understand it, EPSG 4326 is the "standard" latitude and longitude ellipsoid used by GPS systems, and EPSG:27700 is the older British system behind the Ordnance Survey National Grid.

But what's the 6.9: mean?

I tried inputting it into ogr2ogr and it just got confused:

{"errors":["ERROR 6: EPSG PCS/GCS code 6 not found in EPSG support files. Is this a valid","EPSG coordinate system?","Failed to process SRS definition: EPSG:6.9:4326",""]}

...and I can't find any reference to what the component parts of an EPSG code mean, only lists of 'standard' EPSG codes that don't have these initial numbers.

Answer

Note: this is part of an answer already given here, but it seems fitting to post it again.

The 6.9 means that the SRS 4326 specified in version 6.9 of the EPSG database, which you can find here.