The only thing I know is that it's not for iambic operation.
A classic circuit and a nice price make this a great project for beginners or for building into extra rigs. Includes speed control, weight control (rarely offered in "simple" CMOS circuits) and sidetone pitch control. A lot of keyer for the price, it features self-completing dits and dahs for standard single-lever keyer paddles. While not designed for iambic operation, the price and reliability make the 1553 a very nice introduction to electronic CW keying.