The Siemens 6GT2390-1AB00 is a spare-part M12 4-pole plug (socket) used to supply power to a range of SIMATIC RF and MOBY read/write heads and communication modules — specifically the SLG/SIM 8x, ASM 824, ASM 850, ASM 854, ASM 724, ASM 754, ASM 424, ASM 454, SLG D1x, RF640R, and RF670R. It is the power-side connector, not the data cable.
Because it is a spare part for a broad installed base of readers (ASM 8xx, ASM 7xx, ASM 4xx, SLG D1x, RF640R, RF670R), demand is steady but not high-volume. If you are stocking spares for multiple reader types, this single plug covers them all.
What the description tells you about fit
The M12 4-pole form factor is standard for industrial sensor/actuator connections. The socket end mates with the reader's power input; the other end terminates to a 24 VDC supply cable. No special tooling needed beyond a standard M12 wrench.