House Marik: The Free Worlds League, pg 114:
"Twenty-four different 'Mech types are currently manufactured in the League, with the most common being Stingers, Wasps, Orions, Griffins, Hermes IIs, Locusts, Wolverines, and Warhammers.
Same source, pg 115, table listing 'Mech production by chassis. The Wolverine was produced at two different locations with a total production of 36 per year in 3025. Only Wasps and Stingers have higher production numbers.
Same source, pp 88-104, regimental briefs. There are 55 FWLM regiments (not counting mercenaries) listed, and a total of 22 have given 'Mech weight of "Medium," and 15 are "Light" with 16 "Heavy" and two "Assault." Assuming that there's a 3:1 ratio of X:Medium 'Mechs (where X is either Light or Heavy in their respective regimental types) in a lance, then you've got about 50% medium-weight 'Mechs in the FWLM. This isn't conclusive for the WVR, I know. I'm just establishing the FWLM as predominantly medium-weight.
And finally, the coup de grace, from TRO:3039, pg 290:
"A common sight on the battlefields of the Succession Wars, the majority of Wolverines reside in the FWLM. . ."
And
"The WVR-6M became the mainstay of House Marik's medium BattleMech forces during the Succession Wars."
And just for kicks, from TRO:Project Phoenix, pg. 32:
". . . with critics noting that the FWL (a prolific Wolverine user) haven't instigated an upgrade program."