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Cassimer Widmer and Sons of Clerkenwell, London, were well known overlay makers for several UK manufacturers in the early 1920s, notably Conway Stewart. However, by this date, their overlays were much less ornate than the one pictured in the ebay listing.
The hallmark described by the seller dates to the period 1893 to 1913, and his attribution looks to be correct. Similar very ornate high quality overlays are seen in the early years of the 1900s on British pens, typically on those made by De La Rue/Onoto, George Shand or Burge, Warren & Ridgley. I donít know enough about Waterman to rule them out completely, but I think they are unlikely to have been the original manufacturers of the base pen. The date is likely to be between 1900 & 1910, thatís about as close as you can get given that the date letter of the hallmark is apparently badly rubbed.
Hope this helps,