The Matilda CS version is a Close Support 3-inch Howitzer with a L/25 barrel (very short)
The PzIVH has a 75mm L/48 tank gun.
These two weapon platforms have night an day applications in warfare. One is for infantry support (MG Nest / Bunker busting) and the other is for anti-vehicle (as well as a little Infantry support). Only in extreme emergency would they duel one another. This is because the British crew would know it's out classed.
As Peter explained above the Matilda II with the 2pdr would be a better match.
Also any To-Hit value under 0 is a "no shot". (Can't roll a negative number <grin>)
The OM factor is figured from several qualities of a weapon system. Essentially, what makes the gun and fire control do it's job. Each weapon system quality is inputted into an algorithm to produce the OM1, OM2 and OM3 stats. Qualities include stabilization, sights, traverse mechanism, barrel length, rate of fire and others.
Honestly, sources for these weapon systems can vary greatly even for WWI systems. We are all human and not perfect. So opinions vary and everyone has a belly button on the performance of any given system. One of the outstanding qualities of ODGW is that we are open to and encourage those opinions. If you feel strongly that a weapon system is out of line present your sources and we can review them. If we agree then we can update the values. Also if there is a weapon system not listed in a databook, send us the references to that and we can add it.
Lastly, if you believe the values to be wrong or a rule is not as precise as you think it should be, write your own rule. House rules are encouraged and hopefully shared with the community here for discussion and others might use it as well.
Hope this helps