Eastwood appeared in ONE episode of Death Valley Days, namely "The Last Letter." I have it on laserdisc. Clint plays a down-on-his-luck gold miner who never gets any mail ... until one day someone is trying to reach him about an inheritance that he has to travel East to claim before a deadline. It's been a long time snce I watched it but I think that's how the plot goes.
He'd grown the mustache for a part in Wellman's Lafayette Escadrille, but as it happened, a casting change in the lead role left him with a much smaller role than the one that had originally been offered him. (This is according to Schickel's Clint Eastwood, p. 94.) This was in 1957, when he was still a totally unknown struggling actor, two years before Rawhide.
The laserdisc includes four episodes of the series. Besides "The Last Letter," they are "Deadly Decision" with James Caan; "The Kid from Hell's Kitchen" with Robert BLake; and "Three Minutes to Eternity" with Forrest Tucker.