This example is actually stamped 1925 on the motor plate, but the 6 was the same through this period, some models marked No. 6 show up with tin wheels but we believe these were really sold in sets as a No. 5. 

Circa 1928 - This is a No. 6 that we know is from 1928, at least the date on the motor is 1928. The No. 6 from 1927 looks just like this except it has tin drivers.

Circa 1929 - This looks like the No. 00 or No. 66 from 1930 but it's numbered No. 6, The No. 6 below has an American Flyer motor.

Although the number has worn off - you can see the Ives stamping and we know this is a No. 6 with the small casting with drive rods. This is probably 1929 as it's 100% Flyer except for the markings.

This one is 100% American Flyer except for the IVES plate under the cab, this is one of two we've seen this way and we know it's a No. 6 as it has the drivers.

The motor on this No. 6 was originally marked 1928, then marked again as 1929. It looks more like a No. 1 from 1926-1927. This model and has no drive rods or die-cast wheels.

This looks like a No. 17 with the longer cab (extra dome) and front headlight - date unknown, but it's definately rubber stamped as a No 6. Below is another one like this, but this one has a slider bar which was only done in 1930. Normally 1930 locos would be numbered 66, but here are two anomolies.