It is a simple 3-digit panel meter. An accuracy of 1% for an input between 50 mV and 1 V is claimed. However, I can't believe that because of its simplicity. The A/D circuit must be really sensitive to thermal drift and have noticeable non-linearities due to leakage currents.
Because there are no latches between the 7490 counters and the 74141 decoder/drivers, you will see it count through every measurement cycle. Now in 1973, this might have been considered cheap and unelegant. But for the real nixie lover, it means that the tiny little tubes will show their most charming feature, digits merrily dancing to and fro.