The 300-D in:

'data/mekfiles/meks/3039u/Hussar HSR-300-D.mtf' - (large laser variant of the hussar) has fluff that includes:

"deployment:The 300-D variant emerged as a result of a clandestine re-arming agreement with the Draconis Combine, featuring a GM-240 XL engine and an LB 10-X autocannon for closer combat engagements, alongside a significant armor enhancement."