Blandon is outside soaking his shirt when Mark gives him a drink, but a few seconds later when Lucas calls for him to come in the house, Blandon's shirt is completely dry.
After Mark runs off after seeing Blandon's injuries, Lucas is talking with him and, at one point in the conversation, calls Blandon Bill Blandon. All through the story everyone refers to him as Frank.
This is probably evident in the entire series. In many scenes the "W" on the rear pocket of Lucas' blue jeans identifies them as being Wrangler brand blue jeans. Wrangler jeans weren't introduced until 1947, they surely didn't exist in the 1880s.
In several scenes Blandon's "missing" arm/hand can be seen moving under his shirt.
Blandon's arm is not missing. It is kept at his side because it was rendered useless after being wounded in the war. Shortly after approaching Mark, his hand is show in a close-up shot of a large hole in his shirt.
Sheridan tells tells Frank Blandon that "I know a white-haired college president named Robert E. Lee who would be proud to shake your hand." But The Rifleman is set in the 1880's, and Lee died in 1870, as Sheridan surely would have known.