Royal Marine Certificate of service S-1241 or R-138

I have the certificate of service of my father in law SG Deadman CH/X 1872. All details are perfectly clear with one exception that although clear is causing a bit of a puzzle. He joined on a C.S. Engagement.
The date of birth is correct as 11 June 1918 but after that is as follows (18-10-23). Same writing, same hand.
We have no idea as to the relevance of this date. Any ideas?
TIA Bern

 

Hard without seeing it in context..... if you can post a copy of the image of the document you're referring to I'm sure that'll help folk have a better chance of helping you.

 

This should help any experts!

 

It's his age on joining 18yrs 10months and 23days.

Tim

 

Of course, that's what it is. thanks so much.
Bern