Vintage B&M Chrono ref. 3901 — Landeron 48 — 18k gold — Need help

Hi !
I own a vintage Baume & Mercier chronograph, ref. 3901, Landeron 48 movement, 18k gold case.
I went to see a Baume & Mercier salesperson in their store, but they couldn't give me any information about it…

So I'm turning to you:
Are you familiar with this model?
Do you know what year it was made?
Any idea of its value today? and initially ?
What would the price of a service be?

I'll post some photos!
Thanks in advance