Yes, the MT15 came out in 1999, about a couple of years before the MDLP long play modes saw the light.
That makes it something of a venerable antique (in MD terms), and you should at least have mono mode to double your recording time...
For your info (times on a standard 80 minute blank)
(on pre-MDLP machines)
SP = highest quality for standard MD, 80 mins per disk
Mono = same quality as SP, but in mono (obviously), 160 mins
(additionally, on MDLP machines):
LP2 = reduced (but listenable) quality to SP, 160 mins
LP4 = highly reduced quality compared to SP, 320 mins