UK/Commonwealth still has Field Marshalls I believe. I'm sure a troop from there can tell you for certain.
As for insignia placement, well on the collar is perhaps less likely to be seen by a sniper. Better yet is to pull it off entirely in combat.
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"Do not forget your dogs of war, your big guns, which are the most-to-be respected arguments of the rights of kings."
- Frederick the Great, King of Prussia
