When the data sheet talks about the SB370 reverse RMS voltage it is assumed to be a sinewave hence, the peak will be \$\sqrt2\$ higher than 49 volts i.e. 69.3 volts and this pretty much ties in with the peak repetitive reverse voltage statement of 70 volts. They mean one and the same thing for this diode.
Regards the TPSMB36CA limiting at 49.9 volts, this is only true if you operate the device with some form of current limiting (to prevent an excessive current flow) to stop the device becoming damaged.