I just acquired a Septimius Severus denarius which is now listed as Tantalus 26952. In attributing this coin, I noticed a possible omission in the Types for Septimius Severus.
This coin is correctly identified in ERIC as B03,O38, R172, T091 as RIC IV.i 134. The Type description is "Mars standing right, resting hand on shield on captive and holding spear."
However, there are two variations of this reverse. In addition to the sheild resting on a captive, it also rests on a helmet. Here is an example where, although the helmet is not clear to me, I think you can clearly see there is no captive (poossible third variation?): here:
The Sheild resting on helmet is actually the variation listed in the RIC and the captive variant is in the footnotes. The helmet is Cohen 320 and the captive Cohen 321. I found 2 or 3 helmet types online and about 5 captive types. All were listed as C320 (if there was any attribution info given). There is a captive type displayed in Dirty Old Coins as C320.