Most horns hav very small back volume.
In the 15" evs the b was for bass and l was for lead
The b cone is smooth and much heavier than the l
In the 12" the s is for short cone
I don't know if the l was for lead or long
In addition to what Ivan said:
Although most W boxes have a small, sealed back volume, the Don Keele-designed JBL 18" horn's driver chamber was about 5 cu. ft. and vented. Unusual in its day, and now.
The Electro-Voice EVM 15B cone was blacker, thicker, and more long-fibered than its sibling's cone, and had a sinewave-looking cloth edge that on early models was finished with a soft, sticky doping compound, later changed to stiff. The 15L version was grayish, ribbed, thinner, lighter, and the fibers were shorter and more compressed. That cone's cloth surround was sawtooth-shaped and always treated with a latex stiffening compound.
The 12S was created in answer to requests for a "crunchier" midrange performance. Regular-issue 12L speakers have a pronounced mid scoop along with a bit of a higher ice pick, and the 12S's shallower cone profile was found to knock off the peak and fill in the critical 2.5kHz hurt-range some guitarists prefer. Also works out a little flatter overall, but above where most users would cross for PA mids.
All EVM speakers shared the exact same magnet structure, 2-1/2" diameter voice coil and winding length, and spider (within their respective vintages). There was no difference in "throw" as it would relate to linear excursion, at least as far as the motor was concerned. That left you with very efficient midbass transducers (and bass horn drivers) but an 18" model that was, by today's standards for sure, feisty but a bit puny.
There. That is a lot of pedantic minutiae regarding EV drivers and I can't believe I remember it after all of these years.