The Grosvenor (London) auction on April 18 features the John Sussex RDP, FRPSL collection of Machins, which is 245 lots. There are another 100+ Machin lots from other vendors. You can download the catalog for free as PDF files (there are two) from the Grosvenor web site. You can also bid on the web site.
If you are tempted, remember that there is a 20.4% buyer’s premium added to the winning bids.
Meanwhile, I’m trying to figure out why the section titled, “Security Perforated Issues” has so many imperforate stamps.
Update: Not related to Machins, but I found lot 101 to be fascinating. It is a printed sheet, “FRANKS, Wanted by Miss THRING, September, 1828” followed by a list of persons whose frank she wanted. She aimed high: the first on her list is The King. That was followed by a list of dukes, marquises, earls, viscounts and barons. According to the lot description, on the back is written “pray give one of these and any Peer or M.P.”