'LUnit_meters' - What is it?

Hello All,

I am processing some geo-referenced historic aerials images in ERDAS Imagine. These are in;
NAD_1983_UTM_Zone_18N
WKID: 26918 Authority: EPSG

However when I process them (without any changes to the coordinate system) they come out as:
LUnits_meters
Authority: Custom

In ERDAS Imagine these are described as:
UTM NAD83 Zone18
Attachment 32980
Any help is greatly appreciated.

-Samuel

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