Complex sentence: Being the second series of post-war cars, the R12s were very similar to the R10s, except that the R12 was smaller since it was built to meet A Division specifications, while the R10 was built to meet B Division specifications.
Split into simpler sentences: Being the second series of post-war cars, the R12s were very similar to the R10s. However, the R12 was smaller since it was built to meet A Division specifications, while the R10 was built to meet B Division specifications.