This is a common type for Nicaea, but the radiate bust is apparently very rare. The only other example I could find was here. Radiate busts are also apparently very rare among ALL Nicaean types. Did the bust denote a different denomination, as in the Imperial dupondii? Edit: After a bit more hunting, I see that Gordian III and Severus Alexander issues of this reverse type with radiate busts are less scarce.
I don't know what purpose the radiate busts served. Its possible that they represented a multiple (perhaps double) of a local unit. This Volusian from Antioch is similar to the Nicaea issues (what with the standards, and all). Its 22mm, and could be a "two-assarion" coin. Or, it could have served the purpose of the imperial antoniniani, as the size is similar. Then again, there are issues like this monster: It's 30mm and 17.2g, which is imperial sestertius spec; I don't think Volusian had any double sestertii, so no good analog there. Again, its possible this was a double of another unit, perhaps a 8- or 10- assarion, depending on whether Antioch was into the four-unit or five-unit standard.
Interesting post - it provoked me to look through my Nicaea AEs to see if there were any radiates - I have a few, with only one that is supposedly "rare" - a seated Zeus type, rather than the standards reverse: Severus Alexander Æ 18 (c. 222-235 A.D.) Bithynia, Nicaea M AY(P?) X AΛEΞAN[ΔP]O[C]AV radiate, draped and cuirassed bust right / NIKAIEΩN Zeus seated left, legs crossed, holding patera and scepter Rec Gen II.3 p. 472, 582; BMC Pontus -; Lindgren - (2.86 grams / 18 mm) eBay Dec. 2019 Lot @ $1.05 Attribution Notes: Could only find one of these, with radiate bust and Zeus reverse; FORVM listing, noted as "very rare" attributed thusly: Rec Gen II.3 p. 472, 582; BMC Pontus -; Lindgren -; SGICV -, The other radiates I have are the standards type, none for Elagabalus: Severus Alexander Æ 22 n.d. (c. 222-235 A.D.) Bithynia, Nicaea [Μ ΑΥΡ CΕΥ(Η)]ΑΛE[ΖΑΝΔΡΟC ΑΥΓ], radiate, dr. & cuir. bust r. / ΝI−Κ−ΑΙ−Ε / [ΩΝ] between 3 legionary standards (2-2-2 badges; 1-1-1 bars, 1-1-1 ribbons above each). RPC VI, 3260 (temporary). (4.22 grams / 20 mm) eBay Jan. 2021 Lot @ $0.86 Severus Alexander Æ 18 n.d. (c. 222-235 A.D.) Bithynia, Nicaea Μ ΑΥΡ CΕΥ ΑΛΕ[ΞΑΝΔΡΟCΑΥ Γ], radiate, draped & cuir. bust right / Ν-ΙΚ−ΑΙ−Ε / ΩΝ between 3 legionary standards (1-2-1 badges; 2-1-2 bars each). RPC VI, 3201 (temporary); RecGen I-III 617. (3.83 grams / 18 mm) eBay Dec. 2018 Lot @ $0.73 Gordian III Æ 17 (c. 238-244 A.D.) Bithynia, Nicaea Μ ΑΝΤ ΓΟΡ[ΔΙΑΝΟϹ Α...]radiate, draped & cuirassed bust [N]-I-K-A-[E] / ΩN, between 4 standards (1-2-2-1 badges, 2-1-1-2 bars), 2 inner topped with eagles, 2 outer topped with crowns (RPC). (1.64 grams / 17 x 15 mm) eBay Mar. 2021 Lot @ $3.33 Attribution Notes: Very small flan, possible imitative, though style is fine-ish. Bust seen from front, thus RPC number RPC VII.2 19866 (unassigned); Gordian III Æ 18 (c. 238-244 A.D.) Bithynia, Nicaea M ANT ΓOPΔ[IANO]C A, radiate, draped & cuirassed bust right / N-IK-AI-Ε / ΩΝ between three standards decorated on top with wreaths and Aquila. RG 716. SNG von Aulock 653. (2.17 grams / 18 mm) eBay Dec. 2019 Lot @ $1.05
Maybe the OP coin is scarce rather than rare? Have one radiate Æ Assaria, Severus Alexander Bithynia, Nicaea (currently Iznik), 222 - 235 AD 20 x 21 mm, 5.227 g RPC VI 3261 or 3201 (temporary); BMC 101 Ob.: Μ ΑVΡ CEVΗ ΑΛEΞΑΝΔΡΟC ΑVΓ Radiate, draped and cuirassed bust right Rev.: Ν-ΙΚ-ΑΙ-E/ΩΝ City spelled out between 3 standards
I had a Gordian III like that at one time. I don't remember the exact size and weight, but it was around 19-20 mm:
