Several later tube-era Tektronix oscilloscopes such as the 549, 581 and 585 that already use semiconductor rectifiers for the anode voltages contain a circuit that delays turn-on of the main supply voltages until the tubes are warmed up. This is normally not needed in scopes with tube rectifiers because of their inherent soft-start behavior.
A main relay that switches the anode voltages is controlled by a thermal delay relay such as the 6N045 or 6N030 which is housed in a miniature 9-pin glass tube enclosure.
When the scope is turned on, a bi-metal strip in the delay relay is beginning to heat up. After the delay time (30 to 60 seconds depending on the instrument), the bi-metal strip closes a contact that energizes the main relay. One of the main relay's contacts is used to make it self-holding while at the same time cutting out the delay relay, which is beginning to cool so it will be ready to provide delaying turn-on again in the event of a power outage.
The part no is decoded as: 6 - Heater Voltage, NO - Normally Open. XX - Time Delay. For eg. 6NO30 is a 6 Volt filament/heating element with a normally open contact and a delay time of 30 seconds.