The Atlas 6" Models 612 and 618 came out in 1937 and were built mostly unchanged until 1972. From beginning to end, they had Timken bearings and a 1"-10 spindle nose thread.
The Atlas built/Sears Craftsman branded 6" Model 101.07300 came out in 1938 (catalog year so actually late 1937) and was built for one year. It had sleeve bearings and a 3/4"-16 spindle nose thread.
The following year (1939 catalog) it was replaced by the Atlas built Craftsman badged 101.07301. It kept the bronze sleeve bearings but the spindle size was increased to the same as used in the 618. However, for reasons known only to the dearly departed the threads were 1"-8, not 1"-10. Otherwise, most parts were the same as used in the 618. It was built up until 1957 basically unchanged.
In 1958, the 101.07301 was replaced by the Atlas 618 except with a Craftsman badge and Model Number 101.21400. This model was also built until 1972.
There are a goodly number of 101.07301's still in service. So a 1"-8 threaded chuck can eventually find a home.
You will find floating around the Internet cases where someone writes that some 618's have/had 1"-8 spindle nose threads. This is the sort of disinformation that the Internet is infamous for.
Robert D.