Shimano PD-M737 1st generation SPD pedals

In 1990, Shimano introduced Shimano Pedalling Dynamics ("SPD"). A shoe and pedal concept featuring pedals with a retention system and shoes with proprietary shoe cleats, hidden in the shoe tread. It was designed for MTB bikes and riders in the first place, but of course, it was adopted by cyclocross riders as well, although that took a while, since CX racers are quite conservative. At that time, I just joined Shimano Europe company and I was one of the first to test this system. Because I was much more a CX than an MTB rider, I was one of the first people in the cyclocross scene to use SPD. It was hard to convince die-hard cross riders instantly, but after a year or so, most people made the change.
Because of the walking comfort of SPD shoes with completely sunken cleats, it was well accepted by gran tourismo riders as well. Several years later, SPD was extended to the road racing scene. But Shimano had a very hard competitor in Look, a company that already had very good road racing pedals, had already set the standard and even produced clipless road racing pedals for Shimano. So, SPD was far less successful for road than for off road use. The first set of pedals in the SPD off road range was PD-M737, a set on Deore XT level. It features a cartridge bearing system that is quite good (apart from the plastic collar that cracks easily). The foot retention system is two sided, so you can get into your pedals at two sides (top and bottom). Both the front and rear end of the pedals can pivot and are spring loaded. Hardened steel clips keep the cleat in place. The small, steel cleats (SM-SH50 or multi-release SM-SH55) can be clicked in by just putting the feet on any side of the pedals, finding the correct position and pressing the cleats into the retention system. After some practice this can be done really fast, without watching and without failure. To disengage, you twist the heels sideward and the cleats open the spring loaded retention system.
Advantages of the SPD system are: pedalling with feet 100% engaged with the pedals, so you can push and pull the pedals with full power, without the risk of accidentally releasing. On the other side, it's very easy to get out of the bindings if you want or have to. The shoes and pedals shed mud and other dirt easily, so it's really off road proof. Of course, there are some minor cons. The biggest disadvantage is the mixture of yellow sand and mud. This can really clog up and make it hard to engage or release. Later versions of off road SPD pedals and cleats have improved this, but it's always a bit worse than the fool proof Time ATAC system. The 1st M737 pedals and SH50 cleats were subject to wear. When the cleats or the shiny parts of the pedals were worn out, it happened that you couldn't get out. This has been solved by using better material and making spare parts available. It's still recommended to get new cleats every now and then.