In the 60s when Celestion first started producing ferrite magnet speakers they had 3 varieties based on magnet size, G12L, G12M, G12H. The G12L is really inefficient, has very low power handling and is generally the least suitable as a guitar or bass speaker for modern playing hence why its the only vintage greenback speaker that hasn't been reissued. It has a sound that's not bad, but there's a reason its failed to garner the religious devotion of guitarists that the original M and H have. Enter the G12S. I had never heard of this celestion until I found them lurking in my '62 ac30 about 15 years ago. The original Blues had blown or been stolen years ago and the seller told me these were the later silver painted alnico speakers with no bell covers. I knew that sounded fishy, but the price was fair for the amp even with the wrong speakers so I bought it anyway.
Long and short of it, they sound really good. While the letters of the standard series ferrite celestions stand for Light, Medium and Heavy (magnet weight), I believe the S in G12S stands for Special (as in special order) and were supplied as an OEM product for amp manufacturers (like Watkins and Selmer) who wanted something cheaper than an M, but more durable and powerful than an L. This is probably why you never see the S speakers with the famous green magnet covers of the era, hey were left raw so the amp builder had the option of badging them themselves. At this time Celestion was facing their first real competition since the 50s from Fane who were supplying affordable ferrite speakers of very high quality to Hiwatt and Sound City. The G12S was clearly a bid from celestion to keep all their existing OEM contracts.
The G12S sounds like a slightly punchier Blue Alnico with a different compression character when pushed. The G12S speakers are ceramics (aka ferrite magnets) that are somewhere between a G12L and M in construction. Mine have the old pulsonic cones and the cone appears to be the same exact one as the 60s Ms and Hs I've owned. The S speakers are said to handle 15 to 20 watts. I'm not sure what to say about these other than to point out that they are around and command lower prices than G12M's with pulsonic cones (and are even comparable in price to a brand new scumbuck G12M copy), so I would highly recommend buying them as long as they can meet your wattage requirements. They have a chimier top than a Greenback evne when pushed, but like the Blue they are never harsh. When you get the cone to break up it produces that great pre-rola celestion cone crunch you know from so many 60s and 70s records. Scoop yourself a pair or Quad of these while they are still under $500 a speaker, prices are rising, people are catching on. Its hard to describe tone, but I really quite like these speakers as a compromise between an alnico blue and a 25watt greenback.... they really sound the way I wish that a V30 sounded....