Ricoh manufactured a series of Twin Lens Reflex Cameras in the 1950's. There were two basic lines, the first was a series of low cost TLR's that were made out of component pieces of stamped metal. These cameras are easily recognized by their geared focusing rings. These was sold in huge numbers by Ricoh. The other line of TLR's was the Ricohflex "Diamond" Series, started in the mid 1950's in an attempt to compete with the Yashicaflex line of TLR's. The Diamond series was also referred to as "Dia" and "Diacord" cameras. "Diacord," was the name given to the Diamond camera that was exported outside of Japan - an abbreviation of Diamond and an acknowledgment to the top selling Rolleicord series of cameras. Below is a table of the Diamond Series of cameras sold by Ricoh. Data comes from Ricoh's Japanese website. I was surprised to read that the first three models had 3 element Rikenon lenses - I had always assumed they were all 4 element Tessar type lenses. Another surprise is the fact that the only model with an F3.2 viewing lens is the Ricohflex Diamond L model - the rest have a F3.5 viewing lens. Lastly, as you can see in the table, many models were made in a short period time (1955-59 ) and I have seen some versions that have variations not listed. I assume these hybrids are transitional models.
Ricoh Anastigmat 80mm F3.5 ~ 3 elements, F3.5 viewing lens, Citizen-MXV shutter to 1/400. Also called "Dia"
Ricohflex Diamond M
Rikenon 80mm F3.5 ~ 3 elements, F3.5 viewing lens, Seiko shutter 1/10 to 1/300. Also called "Dia M"