Don't know if this helps at all — an RLM directive intercepted in August 1940:
CX/JQ/231
3. On 6/8 following memorandum was received by Luftflotte 3 from Air Ministry:
(a) Different types of Bf 110 D are to be distinguished by numbering, thus:
(1) Bf 110 D-0 has two jettisonable wing tanks and a fixed “basset-belly” (DACKELBAUCH — i.e. tank).
(2) Bf 110 D-2 has two external containers (with choice between bombs or “basset-belly”, as preferred).
(3) Bf 110 D-3 has external containers which can only be used for bombs — no “basset-belly” possible.
(Provisionally this type is also known as “Bf 110 D-0 — B”)
(b) Staffeln are to be equipped wholly with type 110 D-2 or wholly with type 110 D-3, so that they are homogeneous for operational purposes.
(c) Attention is again called to the order that Bf 109 may be used as bombers only after express instructions from LW Führungsstab.
In so far as training “with the new type” has not yet been carried out, a Kette at a time is to be attached for instruction by arrangement with Luftflotte 2, to Gruppe 210.