Those are tests points that you connect a voltmeter to. They are used in setting the bias. You don't need a bias probe, although you can still use one if you want.
If you can get hold of a service manual, the procedure to set the bias is outlined there. I don't have one for your amp but if you look, there are manuals for other tube based Trace Elliot amps available. Reading the procedure can give you an idea of what to do. For the V8 (400W) bass amp for example, connect the voltmeter to the zero test point (ground) and to a bias test point and adjust to 0.3V. After reading the procedure for that amp, since your amp is currently properly biased, you can connect the meter and see what your bias should be set for.
Official Ampeg Portaflex Club #89