Complex sentence: It is believed that the 90 mm weapon has been introduced later than its model number would imply, as a captured specimen was marked "Model 97 Light Trench Mortar, Manufactured in 1942, Osaka Army Arsenal".
Split into simpler sentences: It is believed that the 90 mm weapon has been introduced more recently than its model number would imply. A captured specimen was marked "Model 97 Light Trench Mortar, Manufactured in 1942, Osaka Army Arsenal".