Japanese T94 monthly serial ranges after around 16.8 very commonly overlap. In the last four years of manufacture, I have recorded at least 25 months with out-of-range serial numbers such as your example. However, your example is the first one dated 18.1 reported to me. There are several theories as to why out-of-range examples are encountered, but, the most plausible to me is that some examples just took longer to complete than others (hand-fitting of parts or most any other arsenal work reason). Since the Japanese serialized the guns prior to dating them when they were completed, out-of-range examples periodically show-up.
Much thanks for the info! It's not the first firearm that I have that seems to defy what is commonly assumed about manufacturing dates and configurations. We always seem to learn more with the help of great web sites like this.