As for the "scallop" on the Speed Six; right hand side, rear of frame can be rounded or scalloped, meaning they removed some material to help lighten it a bit. Being a 160-xxxxx serial number your Speed Six will have the "Scalloped" frame and the REDESIGNED/updated (in 1974) grip frame also known as the high back frame. The first grip frame (called the low back frame) didn't go over very good with a lot of people who tested it and those who bought it and since Mr. Ruger wanted to make inroads into the law enforcement sales, the redesigned it to give a better feel, thus the new "high back frame".
The Six Series revolvers (Security Six, Police Service Six and Speed Six) values really depend on where you are buying in the US (West Coast, Northwest and Northeast you'll pay more than in the midwest and southern US) and what the configuration is (blued or stainless steel) and other considerations (were they sold to a particular LE department, overseas contracts, special contracts in the US (USPS, military, etc.). Those things can all contribute to a higher price. Also, if there weren't many made, for example the first stainless steel Security Sixes (low back frame 6" barrel - 3 are known of at this time) and Speed Sixes ( low back frame, made in 1974 ) are pretty darn rare and hard to find and will command a much higher price.
What you paid for your particular Speed Six is what I (and I am a collector of the Six series revolvers) would consider to be a fair price these days. The past 7-8 years there has been a pretty good resurgence of interest in the Six series revolvers that has lead to their increases in price.