Well, there actually was a Probus coin included; which makes little sense. I am assuming it was mistakenly mixed in by a previous owner, since the Numismatic Chronicle is very clear that the latest coins were 5 from Aurelian and suggests a burial date of circa A.D. 270. This Probus is from Ticinum and struck in A.D. 278. The coin is very worn and has similar rust spots as many of the other coins from Mildenhall though. I emailed CNG and hope they will have some clarification about the coin, though I don't expect anything this far removed (some 70 years) from when the hoard was cleaned and examined.
Probus
A.D. 278
Ӕ Antoninianus
20x22mm 2.0g
VIRTVS PROBI AVG; Radiate helmet, cuirassed bust left with spear over right shoulder and shield on left.
VIRTVS AVG; Mars advancing right carrying transverse spear and trophy over shoulder.
In ex. PXXT
RIC Vii Ticinum 430