A technicals book would be interesting, particularly if it discussed the locally preferred types (as mentioned up-thread) and had base vehicles to work from to allow a campaign to have its own technicals. Since people have a tendency to want to add everything to their vehicles, some sort of rules for what happens when a frame is overloaded would help.
For Merc, it might also be useful to have civilian armored cars (Brinks trucks) and possibly some construction vehicles like bulldozers or dump trucks.
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Writer at The Vespers War - World War I equipment for v2.2
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