I have seen the "Tea-Kettle" neck on tenors from just before 20,000 (during the Radio Improved Run) to about 29,500. Timewise, this represents production from roughly December 1934 to Mar 1940.
They seem to appear on about 5% of the Tenors I have seen during this serial number range.
Thanks John that info certainly answered my questions! And this quote from Jason Dumars made me chuckle: