Scott Numbering System

Does anyone know when Scott changed from numeric to the current aplhanumeric system? I have old Scott National album pages I'm trying to date. Example J79-J86 used to be 2285-2292. My page was printed before J87 $1 Postage Due was released. Thanks

1940.
The 1939 Scott U.S. Specialized has the old numbers like your page.
The 1940 Scott (all the world in 1 volume!) has the alphabetic prefixes for the various BOB categories, and the following text in the introduction:

The Scott numbering system has evolved over time. As another example from the back of the book. What we now call C1, C2, and C3 were originally cataloged as 520, 521, and 522 in sequence with the normal postage stamps. In the 1930 edition the air post stamps were separated out to the 1300-series and given numbers 1300, 1301, and 1302. Finally in 1940, they became the familiar C1, C2, and C3. The empty gap of 520-522 still exists in the current Scott numbering system!

Thank you John. I have pre-1940 Air Mail pages too, the Zeppelins were 1312, 1313 and 1314.