'silvered' Roman Coins, What Are They?

Yep, Billon, but not Gordian III (not his legend and he isn't listed as issuing this type). Its actually Trajan Decius:

Trajan Decius Ant "Decius on horseback" Trajan Decius AD 249-251 Silver Antoninianus "Celebrating my arrival in Rome." Obv: IMP C M Q TRAIANVS DECIVS AVG - Radiate bust right, draped and cuirassed. Rev: ADVENTVS AVG - Decius on horse riding left, with left hand raised in salute and scepter in other arm. Rome mint: AD 249 - 250 (2nd & 3rd Issues, 4th Officina) = RIC IViii, 11b, page 121 - Cohen 4

A bit hard to read, but the survivign legend reads ...NVS DECIVS AVG.

Thanks Ben, you are a legend in this field!

One thing I am struggling to understand, is how to know which spreadsheet to download from the site you pointed out http://www.catbikes.ch/coinstuff/coins-ric.htm

Wildwinds I can navigate reasonably fine and tesorillo I find very useful.
Is it just lack of understanding stopping me from knowing what spreadsheet to download? It sounds silly but, I only manage to find the correct spreadsheet if the description in wildwind for example says 'fallen horseman' etc.

What 'coin type' should I select from the dropdown and how should I have known?

Thanks very much

Helvetica is a very advanced reference that I didnt start to use until I was familiar with Wildwinds. I first used it to try and get full IDs for coins instead of just the legends (I had a few coins not listed on Wildwinds).

I really wish I had Tesorillo when I was starting out - recognising the reverse legend and design is the hardest thing and it can only come with experience.

when you learn the legends and start recognising the period by the bust style, you can estimate which sheet you need. Helvetica's spreadsheets are bang up to date and are updated when new examples show up, so you can always find a coin (if Helvetica doesn't list it, it isn't listed anywhere). The major problem is when Helvetica hasnt got the sheet - most of them are late empire, so it was lucky Gordian III was listed.

If in doubt, scroll through the page and look at the descriptions. Some lists are for emperors, others for designs and sometimes a design is in a different spreadsheet - for example, a siliqua with a votive wreath isn't listed in the VOT.. sheet, but the Siliquae sheet.

I wouldnt worry about using Helvetica yet - start with Tesorillo, then try wildwinds.If its early empire, you might need some help - they can be quite hard. If you post them, I can point you in the right direction (sometimes - I'm no good with provincial issues, I dont have a reference for them).

I should clarify, if theres a list for it, it has every known example in it. If you find a new example, its put on, so they stay up to date.

The exception being barbarous examples, really. Quite hard to list those.