identifying tranny

Yo im new to AF and I got a question about my 89 civic STD with a D15B1. i bought it with a 5 speed but dont know if the tranny is off a DX or SI or if 5th gear was just added. How can i find out where my tranny came from and if it originally was a 4 speed with an added gear. I do about 4K at 95mph. Do i have to buy STD 23 spline axles or 26 spline ones? Thanks