Fluid reservoir and systems for filling a fluid reservoir of a fluid infusion device

A system for filling a fluid reservoir of a fluid infusion device with a fluid, the system comprising:

• the fluid reservoir having a barrel portion and a plunger portion, with a fluid chamber defined between the barrel portion and the plunger portion, the plunger portion movable within the barrel portion and including an interface having a pair of locking projections that are opposite each other about a circumference of the interface;
• and
• a transfer guard including a first end and a second end, the first end to be coupled to a source of the fluid, the second end including a pair of locking members opposite each other about a circumference of the second end, each of the pair of locking members to couple to a respective one of the locking projections of the interface to couple the transfer guard to the fluid reservoir, each of the pair of locking projections having a locking arm that is spaced apart from the second end by a stem, the locking arm is arcuate to at least partially follow the circumference of the second end, and the second end includes at least one piercing member that defines a fluid flow path from the source of the fluid to the fluid chamber, and the transfer guard is movable relative to the barrel portion to fill the fluid chamber with the fluid.