Any make of 12AX7 (also known as ECC83 or 7025) can be used as a replacement preamp tube in your SWR amplifier. ECC83 is the European number; 7025 is the military number.
Different "cousins" of the 12AX7 will affect gain structure in different ways. The 12AX7 will provide 100-percent gain. Using a 12AU7 would result in a loss of about 40 perecnt of gain. Obviously, less gain will affect how your preamp reacts. SWR amplifiers are designed to operate in a specific fashion, with a specific gain structure that is best achieved by utilizing the 100-percent gain of the 12AX7.