Brass compression fittings. BS 864-2:1983 or BS EN 1254:1988. They conform to both these standards apart from marking: a fitting conforms to the standards with which it is marked.
https://www.bes.co.uk/15-mm-x-10-mm-reducing-set-17736/46303Compression Reducing Set - 15mm x 10mm https://www.bes.co.uk/media/catalog/product/1/5/15-mm-x-10-mm-reducing-set-imported_min_17736_P_1.jpg0.420.42GBPInStock/Pipe Tube & Fittings/Pipe Fittings/Compression/Lower Prices/Pipe Tube & Fittings4167424212414241454241<p>15 mm x 10 mm reducing set.</p><p>Brass compression fittings. BS 864-2:1983 or BS EN 1254:1988. They conform to both these standards apart from marking: a fitting conforms to the standards with which it is marked.</p>BESt range00https://www.bes.co.uk/media/catalog/product/1/5/15-mm-x-10-mm-reducing-set_min_17736_L_1.jpg5056198111382add-to-cartfacet_type:Reducing Setspecial_price:0.420000cat_multi_facet_l1:Fittingsmore_sizes:17734,17735,17737,17738,17739,17740,17741,17742,17743products_related:20394,8748,18256,6921,9084,7621,16444,6962,21385,21031,7550sku:17736FittingsReducing Set17734,17735,17737,17738,17739,17740,17741,17742,1774320394,8748,18256,6921,9084,7621,16444,6962,21385,21031,7550177360.420000No{"Type":"Reducing Set"}ReducingBESt rangeBrassMetric15 x 10 mm