After looking at the internals today I found no serial anywhere; must be a kit then?
The amp is properly soldered, though I will tidy some things up a bit [I'm looking into details with everything in life]...
Looks like everything is in the right place, compared with the pics I found around this site.
I replaced the signal-path from inputs -pot-tube with a direct wire going from inputs to the input tube, since I use a beautifull passive pre-amp for turning the volume up and down.
Am I right about my amp beeing a kit?
Thanks,
Nico

Welcome Nico! Your years of experience will serve us well here!

Yes, the serial numbers are just a label on the bottom of the chassis; I find the heat makes the adhesive give way over time and they curl off. Yours may not be a kit, though if you had to run a wire to the shunt-volume control, perhaps?

Things are falling into place today.
As strange as it was to find no serialnumber on this amp, what I found out today is strange even more.
After buying a 5U4G [the amp came with a 5Y3G] I found out qiuck enough this rectifier would never fit....
The glass was simply too thick for the small space between transformer and socket.
I just found out the transformer was not mounted correctly. The two bolts on the rectifier-side were not mounted in the threads that are inserted in the housing; new holes are drilled more to the front.
I noticed these two bolts to sit not upright, but under an angle; caused by the threaded inserts.
After unscrewing all four screws the transformer could be moved enough to the rear to use the threaded inserts on the side of the connectors.
Now the transformer is sitting more to the back, and the 5U4G fits easily.
So; it IS a kit IMO.
Nico