Part number of the mounting block to suit 644, 4 gang switched socket is:
PDL640RC - Round conduit adaptor 25mm.
PDL640SC -square conduit adaptor 25.5mm x 16.5mm.
Size: 196mm x 84mm x 19mm high. 84mm mounting centres.
For further information, please visit
https://www.pdlglobal.com/data/product_documents/769_datasheet_document.PDF