Scott #390 is a coil stamp. This is not a coil stamp. It is perforated on all four sides. Coil stamps are perforated on only two sides. Re-check your stamp's perfs with a perforation gauge.
Your stamp must be either Scott #331 perf. 12 with a double line watermark, or Scott #374 perf. 12 with a single line watermark. Sometimes you can see the watermark by holding the stamp up to a strong light, like an intensity lamp. The single line watermark is often hard to detect because of the layout of the repeating letters USPS. Only a small portion of several letters is all that can be seen on a single stamp. So if you don't see a watermark, even in watermark fluid, then it is likely you have Scott #374 with a hard to see single line watermark.