Huge thanks to our friend Russell Laviolette (IG: @paroykos) for writing up the following bit of Mugen history!
The Mugen S1 seat, coveted by Mugen aficionados, was likely designed for the NSX in the early 1990s. Manufactured by Esqueleto of Japan, the S1 would find its way into various Mugen race cars.
pic 1: 2 versions of the S1. Photo credit Project onethirty Carlos and Matthew Yu
Most don't know though that at least three versions of the seat exist, increasing the character of this piece even further. The first version (seen on the right side of pic 1 above) are what most are familiar with. Weighing in at 5.6kg the seat utilizes 5 harness holes to allow for the use of a (you guessed it!) 5-point harness. The characteristic three-tone seat utilizes multiple fabrics including suede.
The backing of the seat is constructed of fiber reinforced plastic (FRP). These seats are NOT FIA approved. That doesn't mean they're unsafe. Mugen only used the seats for competition inside of Japan and FIA compliance was unnecessary.
pic 2: version 2. Photo credit Charlie7
pic 3: version 2. Photo credit Mugen Power Japan
The second version (pic 1-left, pic 2, pic 3) was specific for N1-spec EK9s. It was similarly styled in terms of the harness design, but featured a two-tone seat cover. You'll notice though that the waist area differs to more firmly hold a wider-hipped driver (aka they're wider than standard S1s). The seat back, like the "base model" S1 was constructed of FRP.
Last, and rarest of all, is the JTCC-spec S1 found in the Mugen NSX race machine (pics 4, 5, and 6).
pic 4: version 3. Photo credit Jorge Nuñez and Chad Castelo
pic 5: version 3. Photo credit Jorge Nuñez and Chad Castelo
pic 6: version 3. Photo credit Jorge Nuñez and Chad Castelo
The bucket design is similar to that of the N1 seat excluding the harness hole between the legs and construction material (see below). Conversely, the seat cover is more akin to that used on the standard S1. The most notable difference though is the bucket construction material. Despite the weight savings of FRP, Mugen instead opted for carbon Kevlar, adding much needed strength for intense race situations. These seats are seldom seen except for in a few of the most well-built NSXs in the world (and some old JDM catalogs).
*Note: I have intentionally not mentioned the later S1R seat as it differs significantly from the earlier examples previously mentioned.