I'm not familiar with the dark mustard color made by RRP but I collect the Sunburst pattern pots and they have no marks on the bottom. In the link below, it's the third pot to the right.
Di

Here is a link that might be useful: Robinson Ransbottom Pottery Company

I have a few pieces of their Zephyrus line - a pitcher and a a few bowls. EBAY has a few pieces of RR from time to time .

Here is a link that might be useful: Zephyrus

Thanks to you both for your replies. I spoke with a fellow from Zanesville, who was very helpful. He said that RRP went out of business in 2003, and that pieces were sometimes available on ebay. He was familiar with the pattern, and said that it was manufactured toward the end of their reign. He also said that whomever took over the company was supposedly coming out with new pieces, but that they would be in more modern colors.

I guess I'll keep haunting ebay and yard sales. Thanks!