From their website...
"For heavy duty applications we offer our Exclusive L52HD. Invented by Marlin himself, we start with a L-series transmission housing and line bore it to accommodate larger main shaft bearings. A completely rebuilt, late model G54 gear set (1985-1988) is installed into the modified L-series cast iron housing, including the larger G series bearings. This upgrade is ideal for those that want to upgrade to a late style, stronger transmission but don't want the added length. The L52HD is a bolt-in replacement for L45, L50 and L52 transmissions."