Help! Omega overcoil balance hair spring replacement.

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Hello to all,
I'm hoping you guys can help? I'm working on restoring this lovely Omega pocket watch and my primary problem is I need to replace the balance hair spring which I believe is an over coil type. I know with skill it can be worked gently back to shape but it seems I'm as yet not there, I'm going cross eyed!

So the information I'm after is:
What calibre is it?
Serial number #5199299.
Year: 1915
Movement diameter: 44mm.

I confess the the calibre system of identifying a movement does confuse me.
If anyone can point me in the right direction of a parts suppliers I would be eternally grateful (I know this is a long shot). TIA.