This is the HKS .357 magnum speedloader, which fits the following only in .38 Special and .357 Magnum:

• Smith & Wesson K-Frame revolvers: 10, 12, 13, 14, 15, 19, 64, 65, 66, 67
• Dan Wesson revolvers
• Charter Arms revolvers
• Taurus Mid-Frame revolvers: 606, 65, 66, 80 82
• Rossi revolvers: 971, 951, 851, 351