3LR50.

Some sources list the IEC 3LR50 as one of these two, slightly different batteries, others list it as the other:
One of them has flat pads at opposite ends for the connection (I have also seen it listed as 3LR51), the other uses small snaps, 1 at either end.
I think the confusion arises because they are both out of general production but are made by smaller companies using 3 × LR50 cells. Possibly one is the closest that that particular company gets to the other, Look at the details and decide which is the best fitment for your equipment.

Equivalents:

523: (flat ends)
1306AP, 3LR50, 3LR51, 523, A133, A21PX, APX21, EN133A, EPX21, PC133A, PX21, RPX21, V21PX,

531: (snap connectors)
1307AP, 3LR50, 531, A19PX, EPX19, PX19, RPX19, V19PX,

Image
not to scale. Common
Name. Dimensions.
Length × Diameter. Chemical Composition. Nominal Voltage.

523. 49.9 mm × 16.8 mm
Contacts: Flat pads either end.
Alkaline replacement. 4.5 volts.

531. 58.3 mm × 16.8 mm
Contacts: Mini snap connectors either end (making battery longer).
Alkaline replacement. 4.5 volts.

Image not to scale.	Common Name.	Dimensions. Length × Diameter.	Chemical Composition.	Nominal Voltage.
	523.	49.9 mm × 16.8 mm Contacts: Flat pads either end.	Alkaline replacement.	4.5 volts.
	531.	58.3 mm × 16.8 mm Contacts: Mini snap connectors either end (making battery longer).	Alkaline replacement.	4.5 volts.

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Some sources list the IEC 3LR50 as one of these two, slightly different batteries, others list it as the other: One of them has flat pads at opposite ends for the connection (I have also seen it listed as 3LR51), the other uses small snaps, 1 at either end.

Equivalents: