Spring bar part number for SBGN019 (023, 021)

Hi everyone,
Managed to lose one of the spring bars for my GS Quartz GMT model SBGN019. I bought some aftermarket 20mm bars and while they fit, they are a little too skinny so the end link now shows a gap at the case.

I've read that GS uses standard seiko bars. Does anyone know , or know how to find out, what model springbar I need for this watch?

Thanks
James