Conn GL Prefix on a 88h ?!

[fetzNYC]

Hey everybody,
I have a couple of Conn trombones, one Conn 88h has a serial next to the "88h" engraving GL87XXXX
On the outer slide the serial is L7XXXX ( same number, but the G and the first digit is missing).

Does anybody know what this is about ?
Are these horns rather from 1968 ( L...)
Or 1973 ? ( Usually GAXXXXXX) ?

The engraving simply says "Artist Symphony". No fancy stuff.
Redbrass version.

Any help is appreciated. I have a 48h and 6h with the same thing.

[ConnBone18]

http://www.horn-u-copia.net/serial/Connlist.html

G=1970-1979
L= december
8= 1978
7= slide trombone

[Matt K]

G=1970-1979
L= december
8= 1978
7= slide trombone

The thing I like about these codes is that they're so obvious to interpret!