Serial Numbers

[jale]

Hi
I wanted to list my regs to facilitate servicing and I found out that some have serial numbers and some haven't.
-All my Apeks have them:
.the 4 xtx sets have the first and second with the same number,
the 2 ds4 have number.
-the Scubapro:
.the 2 mk20 have number,
.some mk2 have, some don't have,
.the s600 has none
.none of six g250 have it except my only crystal
.the two r395 have it
.the 109s and the 108s have it also
-the few Beuchats lost at the back of the drawer have number,
-my lovely Tekna-mk10 doesn't.
(yes, I know...too many regs )
So just for the sake of talking, do you know why this discrepancy?
Thanks

 

[Newdiv]

Have you looked under the mouthpiece for the 2nd stages?

 

[jale]

i have looked eveywhere, even where I should not have
numbers are sometimes next to the mouthpiece, sometimes on the edge under, sometimes on the exhaust

 

[jale]

my bad...I retract that I said.found indeed some numbers on the mouthpiece exhaust but i needed a good light...
i really need a new pair of glasses
Thanks again @Newdiv