Guys!...
The "Rogers Book", while an EXCELLENT tool/read, is inaccurate on it's Dynasonic datings. That said, serial #18860 is 1969/Dayton as 1968 ended with serial numbers in the 17000's... give or take of course! Could be "close enough for jazz" as the saying goes. 1969 is more accurate though. Also...
It's not a "ladder"... it's not a "rail"... and it's not a "cage"... it is correctly referred to as the "snare frame", and that is what Joe Thompson/Rogers called it!
Here's a pic of my 1968 WMP Dynasonic, serial #14080. While not a 61/2X14.. ( which is an incredible find! ) .. it is near MINT!
Tommyp