Yes, the SD always wore a blank collar patch.
As for the Sig-runes: These were never
used by the Allgemeine SS, but rather exclusively by the full-time, armed, paramilitary SS units. The development of the runic collar patches roughly breaks down like this:
1934: The members of the early full-time armed SS security units, the Politische Bereitschaften
, started to wear collar patches with the runes and an additional arabic numeral (1, 2 and 3) or, in the case of the Leibstandarte, just the runes.
1935 - 1940: From the Politische Bereitschaften
developed the pre-war forerunner of the Waffen-SS, the SS-Verfügungstruppe
which used collar patches with the runes and an additional number, letter or symbol, while the Leibstandarte continued to wear just the runes.
1940: An order of 10 May rendered the collar patches with runes and additional numbers, letters, or symbols obsolete. From this date on, just the runes were to be worn by the Waffen SS
, a term which became official in this year. These patches were universally used by the Waffen-SS until the end of the war. (With the exception of units who were assigned other collar patches, such as the Totenkopf-Division or units composed of foreigners or ethnic Germans, but that's a whole different study.)
See also these threads:
http://www.thirdreichforum.com/phpBB2/v ... hp?t=13978
http://www.thirdreichforum.com/phpBB2/v ... hp?t=11484
http://daggers.infopop.net/2/OpenTopic? ... 6233083196