Mk 10 is standard freefall
Mk 18 is retard version
Mk 20 has airburst fuzing – possibly retard version
Mk 13 is probably low drag with airburst

Not true! All are the same shape and almost the same weight (1000lb). Slight weight variations result from different fillings and insulation etc; all are interchangeable as such and the role is decided by fuzes and tail units. However 10s are Naval surplus (ex early-Buccaneer fuzing system) and should be in very limited stock (if any left) 13 and 18s should have been phased out. 20s and 22s are the later variants whilst 15s are concrete filled for practice (have been used in earnest to prevent collateral damage.
:).

PS, SDB is a freefall bomb per se and should not be compared to Brimstone