For what its worth, from AS 2469 (Two Piece Gas Cylinder Standard)
S11 SURFACE COATINGS
The surface coating used on the cylinder shall be identified by the symbol shown. The
symbol shall be an Arabic figure within a circle of 10 mm minimum diameter, located in
accordance with Clause 13. Where required by the application code under which cylinders
are to be used, one of the following surface coatings shall be applied.
(a) Hot-dip galvanized in accordance with AS/NZS 4680, and identified by symbol (1).
(b) A base coating to a minimum dry film thickness of 25 ?m, on a surface prepared by
abrasive blasting in accordance with AS 1627.4, to not less than Class 2½, and
followed by coating with a high grade resin-based material with recognized outdoor
durability to give a total minimum dry film thickness of 75 ?m, complying with either
of the following:
(i) Inorganic zinc-rich coating in accordance with AS/NZS 3750.15.
(ii) Organic zinc-rich coating in accordance with AS/NZS 3750.9.
The identifying symbol shall be (2).
(c) Sprayed zinc coating in accordance with BS EN 22063, to be not less than Class Zn4
for the entire surface area from the internal junction of each attachment for a width of
not less than 25 mm, and to not less than Class Zn3 for the remaining surface area.
The identifying symbol shall be (3).
(d) One of the following, and identified by symbol (4):
(i) Red oxide/zinc chromate primer applied on the surface treated in accordance
with the coating supplier’s recommendations, with a high quality alkyl-based
top coat to a total minimum dry film thickness of 50 ?m.
(ii) Polyester powder coat on a surface treated in accordance with the coating
supplier’s recommendations, to a minimum dry film thickness of 75 ?m.
(iii) Systems shown by testing to have equivalent surface-protecting performance to
the systems specified in Items (i) and (ii) above.
I can see no reference in the 2010 version of AS 5602 (Caravan Gas Fitting Standard) to any specific cylinder surface coding requirements. Earlier versions may have had them, but latest one doesn’t appear to.
Some retailers sell both (2) and (3) surface coded cylinders as suitable for caravans:
http://www.discountcamping.com.au/index.php?page=products&pagesubid=gascylinder&pageid=gasPerhaps someone with a bit more Gas fitting knowledge may know otherwise, but IMO more important to have the cylinder in date and not damaged.