The similarity certainly does appear deliberate. It does seem possible that Constantine (at this time) may have not only wanted to invoke/appeal to Mars, but want to be seen as him. It's interesting to look at the London issues both before/during and after his Italian campaign (attack on Maxentius). In the PLN -|* (star right) issue the MARTI type is notably missing, but we have Constantine himself with aptly-named martial bust varieties. After his victory, in the PLN *|- (star left) issue, MARTI now returns. It's as if Constantine himself becomes Mars for the campaign. In fact we see bust types combining both martial (helmet, spear, shield) attributes as well as Sol's radiate crown, so perhaps Constantine is channeling both Mars and Sol, not just appealing to them for victory!
No need to read anything into it...the similarity of the bust types might just be like all the tetrarchic coins; in which all the busts look a little alike.
There's some truth to that, especially among *tetrarchic* coins, but I there's actually quite a bit of differentiation between Constantine/Licinius and Maximinus at Constantine's mints at this time. I guess Constantine didn't get Diocletian's memo that they were all just replaceable cogs! Anyway, I do think the PLN star right issue supports the suggestion.
I like the series so much that I looked for and found another one from a different mint (Londinium). This is the seller's picture. According to his description the coin has a diameter of 28 mm, which is probably as big as they come. IMP CONSTANTINVS P F AVG // MARTI PATRI PROPVG --- PLN RIC 108.
I like the helmeted series from London a lot. Here is one of mine. I suppose the reverse legend (COMITI AVGG NN ) should probably be read as Sol Invictus companion of the Augusti. What does the NN stand for?
N is an abbreviation for NOSTRI, meaning "our", but it's doubled to NN to indicate a plural to agree with AVGG, so the unabbreviated legend would be something like "COMITI AVGVSTORM NOSTRORVM" (companion of our augusti). I didn't take Latin, so not 100% sure of that full legend. For some reason this reverse legend - despite the inclusivity - was only used for Constantine himself, but there's also the similar COMITI AAVVGG which was issued for all three of Constantine, Licinius and Maximinus, using this rather unusual AA-VV-GG indication of plural rather than the more normal AVGG.
I agree this is a very nice series of large coins from Constantine. Here’s mine from Trier: Constantine I, 307–337 AD BI follis, Treveri, ca. autumn AD 307–end 308. Obverse: IMP CONSTANTINVS P F AVG,laureate and cuirassed bust right. Reverse: MARTI PATRI CONSERVATORI (to Mars the Defending Father), Mars, naked in helmet, standing right, holding spear inverted and resting arm on shield, S—A in fields, PTR in exergue. Reference: RIC 772a. Weight: 6.68g Diameter: 27mm Conservation: Nearly extremely fine