I've always thought the Simplex SX610 was the the econo version of the SLJ 6600 (T/SP versions). However, the SX610 has a max wrap and max cog size of 28 teeth, while the SLJ 6600 has a max wrap of 22 teeth and a max cog size of 24 teeth, according to Velobase catalog scans. Why the difference? Is the pulley cage a bit longer? The parallelogram travel a bit wider?