Send the complete old assembly if you can. Even a broken sample is useful. Keep the stationary seat, rotating head, springs, drive parts and any label together. Mark the orientation before dismantling and do not clean away the wear pattern until it has been photographed.
Include photographs of the seal fitted in the machine, then the removed parts from the front, side and back. Put a ruler beside the parts and photograph any marking. A photograph with a ruler is indicative, not precision metrology, so confirm key sizes with a micrometer or callipers. A colour or a shape alone will not establish the rubber compound or the face material.
Give the shaft or sleeve size, chamber bore and depth, and the space available for the seal. Add the setting length or installed height if you know it, and any drawing of the seal or gland. Include the pump make and model, old drawing or purchase record if there is one. Tell us whether it is a component seal or a cartridge assembly and whether the machine has been modified.
Finally, describe the duty. What fluid runs through it, at what concentration, pressure and temperature? Does it see solids, steam, cleaning chemicals, vacuum or periods without liquid? Are food or pharmaceutical approvals needed? Tell us when the last seal failed and whether it leaked immediately or after running.
If the machine is stopped, give the safe time window for a decision. We can start with photographs and measurements, but may need the sample or a closer check before confirming a suitable part. The aim is not just to find something that looks the same. It is to avoid a second shutdown caused by the wrong replacement.
Related: How do I measure an unknown mechanical seal for replacement? and Seal Troubleshooting Guide.