One of these ?
http://www.radiomuseum.org/r/rca_240_1.html
I picked up an RCA 222 earlier in the year and have been looking all over for another to compare it to with no luck. I've seen a couple RCA 221s for sale recently for about $100 each.
The 240 is a higher end model (more tubes) than the 221 or 222 so I imagine less common and more valuable.