According to Situation maps produced by HG Weichsel, the unit Bgl.Btl. Reichsführer-SS did indeed exist on the East Front in Pomerania in February 1945 and it did have a StuG Kp..
Maps I have downloaded from: ftp://jccalvin.ddns.net/Situation%20Map ... ry%201945/
contain OB information on this unit.
On 6 -7 Feb it was part of Gp Munzel and Stu Gesch Kp Reichsf SS is listed separately under X.SS Kps.
On 07 Feb The StuG Kp is still under X.SS, but part of RF SS Begl Btl is listed under Gr Voight and part under Gr Munzel.
By 10 Feb, the StuG Kp is no longer listed, where it was listed, SS Pz Jg Ab11 is listed. The remainder of the unit is still split between Gr Voight and Munzel.
On the 13th, the unit is only listed as part of Gr Voigt (In Arnswalde).
However, on the Map for 15 Feb 45, there is a Meldung for 11. Armee with SS-Pz Jg Kp RF-SS and a strength of 4(4) on the StuG/StuH/JPz 38 line and status dated for 13 Feb 45.
(AG Weichsel Maps); King Tiger vs JS-2 - Operation Solstice 1945, David R. Higgins; The Waffen-SS and the Struggle for Pomerania, 1945 (Part I): The liberation of Arnswalde - Published in „Siegrunen“ Magazine – Volume V, Number 4, Whole Number 28, January 1982; Tigers in Combat vol 2 (Kindle ed.) loc 4186 (503 sPzAbt) ; Flames of War topic on sPzAbt503 in Pomerania; WarThunder After-action Report Armor battles of the Waffen-SS 1943-45; Axis History Forum, topic: Lets write about operation "Sonnenwende" 1945;Traces of War – Operation Solstice Feb 1945