I've quoted the relevant piece from CoreArmor.xml below. This is the "fallen" model version of the padded armlets, there is also one for normal-sized empire units. This item, for some reason, has a prerequisite for a raceclassification "Men", but this is an empire item and it is only ever gained by empire players. There is no faction in-game that has access to both the technology, and has a raceclassification of "Men" (even the Kraxis have a raceclassification of "Fallen"). Thus, this item cannot be used by anyone, and this means the "fallen" modeltypes cannot train with padded armlets.
<GameItemType InternalName="TrogClothArmlets">
<DisplayName>Padded Armlets</DisplayName>
<Description>Padded Leather Greaves, lined and padded give the wearer some rudimentary defense.</Description>
<Type>Forearms</Type>
<SupportedUnitModelType>FallenMale</SupportedUnitModelType>
<SupportedUnitModelType>QuendarMale</SupportedUnitModelType>
<SupportedUnitModelType>UrxenMale</SupportedUnitModelType>
<GameItemTypeModel>
<ModelFile>Gfx\HKB\Items\Armor\Fallen Armor\fallen leather armor export C\fallen_leather_vambrace_C.hkb</ModelFile>
<Attachment>forearm_left_lcf</Attachment>
</GameItemTypeModel>
<GameItemTypeModel>
<ModelFile>Gfx\HKB\Items\Armor\Fallen Armor\fallen leather armor export C\fallen_leather_vambrace_C.hkb</ModelFile>
<Attachment>forearm_right_lcf</Attachment>
</GameItemTypeModel>
<CanBeEquipped>1</CanBeEquipped>
<CustomizationPointCost>5</CustomizationPointCost>
<IconFile>PatchworkArmlets.png</IconFile>
<TintR>0</TintR>
<TintG>0</TintG>
<TintB>0</TintB>
<ProductionRequirement>
<Type>Resource</Type>
<Attribute>Gold</Attribute>
<Value>3</Value>
</ProductionRequirement>
<ProductionRequirement>
<Type>Resource</Type>
<Attribute>Materials</Attribute>
<Value>1</Value>
</ProductionRequirement>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Defense</StrVal>
<Value>1</Value>
</GameModifier>
<Prereq> <Type>Race</Type> <Attribute>Men</Attribute> </Prereq> <Prereq> <Type>Tech</Type> <Attribute>Tools_Of_War_Trogs</Attribute> </Prereq> <Weight>8</Weight>
</GameItemType>
