could be something like this http://velobase.com/ViewComponent.aspx?ID=e4216060-adf8-44a3-8b9a-1570455057be&Enum=103 although not sure these exact parts were used on non high-end racing frames... maybe just get a band-on downtube shifter boss just above it and mount the normal cable stop adapters...
The original parts would have been https://www.pedalpedlar.co.uk/products/s213-shimano-600-ax-aero-vintage-downtube-shifters
@mattyc started
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could be something like this http://velobase.com/ViewComponent.aspx?ID=e4216060-adf8-44a3-8b9a-1570455057be&Enum=103 although not sure these exact parts were used on non high-end racing frames... maybe just get a band-on downtube shifter boss just above it and mount the normal cable stop adapters...
The original parts would have been https://www.pedalpedlar.co.uk/products/s213-shimano-600-ax-aero-vintage-downtube-shifters