This thread is an index to various other threads and references on the early JBL 4-way monitors.
From the L250 profile on this site:
The four-way concept allows for very wide dynamic range since the individual drivers are called on to reproduce a relatively narrow range of frequencies and therefore the overall system can handle more power. Within their narrow range, each driver can be fine tuned in the network design to result in an inordinately flat response ...a 4-way concept that Greg Timbers pioneered in the 4315 monitor of 1974.
The 4315 is probably the smallest of the "big" 4-way Studio Monitors at 33 5/8 x 20 1/2 x 12 1/2 and weighs "just" 95 lbs!
According to a post from Riessen the very early 4315 are recognizable by the dual ports down low on the baffle. You seldom see this model and they are quite rare. The later versions (4315A and 4315B) had a single port (as below)
4315A image below courtesy of Subwoof (nice looking speakers!)
Middle left shows a set of 4315 drivers - photo probably from a Japanese 43xx fan site
Middle right is the early 2 port style 4315 sold via ebay in May 07