Imitations from this period are uncommon. It was probably becoming harder to profit from making silver-plated coins when the official pieces were alredy rather low in silver content.
Trajan
Decius (AD 249-251)
21 mm. 5:30. 3.89 grams
Yellow brass, without traces of silver. Worn looking, perhaps a cast.
However, the edge shows no seam.
IMP C M Q TRAIANVS DECIVS AVG
/DACIA, Dacia standing holding staff with "draco"
military standard.
RIC 12b, page 121, "249-251".


21 mm. 12:00. 2.14 grams. Remarkably thin
fourre.
Its prototype.
IMP [C M Q TRAI]ANVS DECIVS AVG
/DACIA, Dacia standing holding staff with "draco"
military standard.
Prototype: RIC 12b, page 121, "249-251". AR22. 6:00.
19-18 mm. 6:00. 2.16 grams
Hybrid. Distinctly small. Little silver remains, but clearly fourre.
IMP C M Q TRAIANVS DECIVS AVG
/VIRTVS AVGG, Virtus standing right holding vertical
spear behind
Reverse type of Philip II or Treboniainus Gallus, but not Decius.
Prier IV (6/26/1998) lot 310 was from the same dies, and the same size.
It was a very similar coin but for the weight.
Trebonianus
Gallus (AD 251-253)
24 mm. 12:00. 3.36 grams
A larger (!) flan than the official pieces, but thin.
IMP C C VIB TREB GALLVS AVG (poorly struck, so it
is hard to read, but it is there)
/female figure seated left holding out arm with
? and vertical sceptre
/legned illegible, maybe with VI and ending AVG
A common obverse legend of Gallus.
20 mm. 5:30. 3.53 grams.
Legend correct but lettering unevenly formed -- clearly irregular,
but a very nice example.
IMP C C VIB TREB GALLVS AVG
/PAX AETERNA, Pax standing left holding olive branch
and sceptre (or cornucopiae)
Prototype: Sear 2788. RIC 71, page 166, "mint of Milan" undated
251-253.
20 mm. 5:30. 3.38 grams.
IMP C C VIB TREB GALLVS AVG
/PIETAS AVG, Pietas standing, raising both hands. Altar at
feet left.
Prototype: Sear 3.9242 (no photo), RIC 72 (Milan) page 166, plate
13.16.
Gallus is pretty late for silver-plated imitations. The profit must have been diminishing -- there was less and less silver in the official coins!