Hi Stig.
Sorry you're not getting many responses, but frankly I doubt we can help you much with one exact date. The amp serial number and appearance don't seem to fit together in that the serial number looks a lot earlier than the amps faceplate would suggest. See here (including the dating page near the bottom of the left menu) for an idea of what I'm talking about:
http://users.telenet.be/orangefg/" onclick="window.open(this.href);return false;
I'd say you probably have an older head chassis which was later put into a combo cabinet. Hard to say for sure exactly what is what though. Are those the original transformers for example? Don't know...
That particular link to Celestion date codes isn't the best. The info is badly / unclearly laid out.
Go to Celestions own site here:
http://professional.celestion.com/guita ... /index.asp" onclick="window.open(this.href);return false;
and choose 'chassis date codes' from the bottom drop down menu. You'll see better which columns are for year, which for month.
Since all codes from 1968 to 1991 are written in the form: Month, Year, Day, that makes your DK25 driver 25th April '77. Who know if it is original to the amp / combo cab though. Soldering certainly isn't original...
Andy.