Sestrel Compass ? Top up

[Guest]

I Have purchased what I think is a Sestrel grid compass from a local car boot sale.
It is black , 6" diameter & marked "Heath Marine , London England " on the card.
It has very large air bubble in it - does any one know how to top it up ?.
I cannot see any screw to do this unless it is hidden under the bezel.

Bernard

 

[dickh]

I beleive this compass is no longer manufactured, but try S.I.R.S Navigation Ltd. lon Tel 01322 383672 They are in Swanscombe in Kent and can probably service it - they are the service agents and now manufacturers of the Sestrel range.