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

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.

The question has an accepted answer, but for completeness, the OGC document

Definition identifier URNs in OGC namespace, 07-092r1, 2007

Tells us that:

To identify the definition of a single object, the URN shall have the form:

urn:ogc:def:objectType:authority:version:code

the URN value for an anyURI that references one object in the European Petroleum Survey Group (EPSG) database shall have the form:

urn:ogc:def:objectType:EPSG:version:code

So 6.9 here is the version of the authority definition.

Note that:

The urn, ogc, def, and six: parts of this URN are fixed.

'This' references the URN for the definition of a single object, that means that urns without the optional version should have a double colon in them like:

urn:ogc:def:crs:EPSG::27700