When the list of suspects is being typed for Blore, the order reads, "Judge L.J. Wargrave. Anthony Marston. Philip Lombard." ... so forth. When Blore makes notes next to the names later, "Thomas and Ethel Rogers." comes before "Anthony Marston."
When we see the envelopes of the letters to the Ten Little Indians being franked, it appears that the sender (rather than the postal sorting office) is doing the franking. Also, the franking mark is made entirely on the envelope itself, when the reason for franking is to overlay the stamp with a franking mark to prevent it being reused.