Yes it is quite right it doesn't have the letter. I think it's easy to go blind looking through 300 almost identical coins!
I can find 10 coins with a star in the left field of the reverse and no letter on the obverse, and Constantius in a pearl diadem. 8 of them are AE2 and 2 are AE3, so knowing the diameter would help. All of them have:
Obverse: Bust of Constantius II, pearl-diademed, draped, cuirassed, right; D N CONSTAN-TIVS P F AVG
Reverse: Soldier, helmeted, draped, cuirassed, advancing left, spearing fallen horseman with right hand and wearing shield on left arm; shield on ground to right; star in left field; FEL TEMP RE-PARATIO.
Apart from the coin size, the difference is on the reverse - whether the horseman is wearing a (pointed) cap or not, and whether he is falling forward clutching the horse's neck like this or facing the soldier and extending his right arm like this. I think it is clear he is extending his right arm on your coin, which narrows it down to four coins:
Large AE2 (about 25mm):
RIC VIII Antioch 122: Horseman wearing cap, facing soldier, extending right arm; AN(A/B/Γ/Δ/∈/S/Z/H/θ/I)
RIC VIII Nicomedia 62: Horseman wearing cap, facing soldier, extending right arm; SMN(A/B/Γ/Δ/∈/S)
AE3 (17-21mm):
RIC VIII Nicomedia 98: Horseman wearing cap, facing soldier, extending right arm; SMN(A/B/Γ/Δ/∈/S)
RIC VIII Cyzicus 113: Horseman wearing pointed cap, facing soldier, raising right arm; SMK(A/B/Γ/Δ/∈/S)
If your coin is about 25mm in diameter, you cannot tell the difference between RIC Antioch 122 and RIC Nicomedia 62 without the mintmark. But Antioch 122 appears to be the more common coin, so it is probably that (Antioch is ‘scarce’, Nicomedia is ‘rare', and Ocre has several examples of Antioch but none of Nicomedia).
I think your horseman does not have a pointed cap. So if your coin is 17-21mm, then it is possibly RIC Nicomedia 98. However, it is not easy to see if the hat is definitely not pointed, so maybe it is RIC Cyzicus 113, which is by far the more common coin (Cyzicus is ‘common’, Nicomedia is ‘rare', and Ocre has several examples of Cyzicus but none of Nicomedia).
None of these are on Numista. Your coin has no mintmark so can't really be the example, but you can add a page.