@Sid never seen a hunchback like that nor the other mech.
Record Sheets 3039 has the CPL-K2 which is shown on the Tactical Operations cover (behind the Uziel's legs)
The quote from Sarna:
CPLT-K2 - Breaking the mold of the Catapult, the K2 model removes the LRM-15 launchers and replaces them with two PPCs which allows the Catapult to act as a direct fire support 'Mech and take a more active role in front line combat. The 'Mech has additional heat sinks to help dissipate the added heat from the PPCs. The K2 also has two Machine Guns to deter infantry attacks. BV (1.0) = 1,052, BV (2.0) = 1,319[9]
I'm not too familiar with the other variants (although I've heard of the Plasma Rifles and Heavy PPCs), and Sarna doesn't have proper citations for them aside from the Master Battle Value list, but there are variants that use ER PPCs, Heavy PPCs, and Plasma Rifles see -
http://www.sarna.net/wiki/Catapult#cite_note-8You'll note that these Catapults don't have any missles and are designed to fight up close, rather than be simply fire support.
Likewise, there's more variants of the Hunchback than I realized- most of them in 3039. In addition to the one that drops the AC20 for Medium lasers, there's also one that drops the AC for LRMs (4J, making it long range fire support than up close and personal), one that replaces the AC with SRMs (4SP), and one with MMLs (5SS). I've seen the 5SG used, although according to Sarna it never entered full production (switches the AC for a Gauss Rifle)
http://www.sarna.net/wiki/Hunchback#cite_note-MUL111-9All those variants are pretty significant changes, and are on par with your friend's Thunderbolt variant...except these are official variants that are made in universe by the companies.