It's probably a slightly different shape designed to force a more positive seal as pressure increases. More than likely R and Rx are interchangable. Now you can call them and verify what I just told you.
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I have got the same response above from supplier that both are identical. But, I would like to know which one used for better postive sealing. Can we use R 73 gaskets on RX groove??
Both R and RX grooves have typical dims. The height of an R-73 gasket is 0.69", RX 1.250". Using an RX gasket will increase the standoff between the two flanges, which will also increase the OAL length of your assembly. The studs/bolts used to connect the flanges will increase using RX verse R gaskets. RX gasket provides a pressure-energized self-sealing effect.