Complex sentence: The class series, however, was not yet incorporated into the number, and numbering of variations within the class was still sequential, thus the Class 10E1 was still numbered sequentially following the last Class 10E number (10-051 to 10-100).
Split into simpler sentences: The class series, however, was not yet incorporated into the number, but followed the old practice (10E, 10E1,10E2). Numbering of such series within the class was still sequential, thus the Class 10E1 was still numbered sequentially following on the last Class 10E number (10-051 to 10-100).