Lo6L39 year?

[hcs]

I have a Lo6L39 serial no. 75972 that I have been restoring for many years. Now I need know the year it was produced, because Dejan will make the serial plate for me. I found a photo of a Lo6L39, with serial no. 76112 from 1940. 

The tubes is all dated from 1943 and 1944, but since it was used by the danish navy, I guess they have been changed...

I didn't find any clues of a date inside.

I hope that you can help me, "guessing" the year it was produced, by looking at some photos:

The serial no. 75972 is stamped in the top:

 

The front:

Front inside:

Capacitors, normal type, not the ceramic type:

Some stamps in the top:

 

I really you can help me out :-)

 

[hcs]

This is the Lo6L39 serial numbers, I have found untill now:

75972  this is mine, without year

76112  40

223  42

282  42

329 42

Lo6L39a:

309  43

393 43

This was found at WAF, it is a quote from Funksammler:

"I suspect that the 1940 numbering system was slightly encoded, so perhaps it's real serial number is 112."

My guess is that my Lo6L39 is from 1940 or maybe 1939, if 76XXX is 1940, perhaps 75XXX is 1939?

Any thoughts about this?