From my research (I first thought they were ceramic capacitors), the TNR appears to be a MOV type, 681 is 680 volts, K is 10%, and then things get vague. I assume 015A is some multiple of 1.5 amps, and maybe 70116 is a date.
The 15 is the series size (in mm) and the type numbers are more often written the other way around these days like this photo of a later Marcon specimen:
The
681 is the specification for "
nominal varistor voltage at 1 mA", so in this case 680 volts. (Well, actually, according to the datasheet 612-748 volts, nominally 680.)
The specs from the datasheet say:
Maximum continuous applied voltage ACrms: 420 V
Maximum continuous applied voltage DC: 560 V
Peak current 8/20 uSec: 4500 A
Maximum energy 2 mSec: 90 J
Rated wattage: 0.6 W
Clamping voltage: 50 A, 1110 V
Typical capacitance @ 1 kHz: 340 pF
Varistor voltage @ 1 mA DC: 680 (612 - 748) V
MOVs generally suffer from degradation over their lifetime due mainly to total surge power clamped over their lifetime, be that slow degradation from repeated small surges or large spikes causing instantaneous destruction. If these are just in the power supply input circuitry for clamping of the mains input, any similar appropriate MOVs should be suitable.