I think the trend toward smaller SLRs was started by the Fujica ST701 and not by the later Olympus M1/OM1. There is always the idea that more than one company can be working on an idea at the same time but the ST701 predated the M1/OM1. When you look at various SLR features you can see that some companies stuck with older technology longer than others. In 1965 Konica had the Auto Reflex. It had shutter priority automation, a bayonet mount, automatic aperture indexing, full aperture metering, a vertical metal shutter and even the ability to switch from full frame to half frame. The one missing piece was TTL metering. This was corrected in 1970 with the Autoreflex T/FTA. The Minolta SRT 101 came out in 1966. It had the interesting CLC metering but still had a cloth shutter and match needle metering. The Canon FTb had limited area metering and the QL feature but still had match needle metering and a cloth shutter. Pentax was the stalwart. The Spotmatic II of 1971 still had a threaded mount, stop down metering and a cloth shutter. Pentax did not have a bayonet mount until 1975, ten years after the Konica Auto Reflex. Between the Pen series SLRs and the OM-1, Olympus briefly made the FTL. It had the M42 mount too. Nikon must have been stuck by a patent problem because its Automatic Indexing system did not apear until 1977. It looks just like the system which Minolta started using in 1966. The exciting SLR (DSLR) features today are very high ISO performance, frame burst speed, built-in HDR, very fast AF etc. If you aren't shooting an Indy race the old equipment can still do a fine job.