Book  (2021)  Page(s) 460.  Includes photo(s).
 
Translation:
'Spek's Improved' ....The DNA-Profile was prepared by Pom [Swedish Programme for Plant Diversity]. ‘Spek’s Improved’ is closely related to ‘Hollandica’. Their similarity index is 90%. Morphologically, it is mainly the number of petals which distinguishes the two varieties from each other. ‘Spek’s Improved’ has 25-35 petals, ‘Hollandica’ has five.

Article (magazine)  (Dec 2015)  Page(s) 22.  Includes photo(s).
 
‘Neuvoksenruusu’ (”the Counsellor’s rose”) (R palustris x rugosa) and ‘Neuvoksettarinruusu’ (”the Lady Counsellor’s rose”), R. x spaethiana, ’Spek’s Improved’. These roses have persisted in old gardens. They were previously used as rootstocks and have survived long after the tender scion perished. They grow into giant shrubs 1,5 to 1,8 m high with narrow, greyish-green leaflets. The flowers of the ”male counsellor” are quite large, single, rosetinted. His female counterpart carries semi-double flowers.

Book  (1986)  Page(s) 179.  
 
Understocks...Hybrids... 'Spek's Improved'

Book  (1981)  Page(s) 113.  
 
R. rugosa hollandica (= R. rugosa x manettii; Boskoop's Rugosa') Various selected forms are to be found in Holland, e.g. 'Spek's Improved'. 

Newsletter  (1975)  Page(s) 759.  
 
Rosa rugosa grown for rootstock is probably a hybrid of R. rugosa x R. cinnamomea. Some selections out of this cross have been multiplied: the it common one is Spek's improved (BOOM, 1949).

Book  (1959)  Page(s) 243.  
 
De bekende rugosa-onderstam is een bastard, waarschijnlijk R. rugosa x plaustris of virginiana; bloemen klein, lichtrose; hiervan bestaan selecties b.v. 'Spek's Improved' (Spek), met kleine, half gevulde bloemen.

Translation:
The well-known rugosa understock is a crossing, probably of R. rugosa x palustris of virginiana; small blooms, light pin; thereof exist selections e.g. 'Spek's Improved' (Spek), with small, semi-double blooms.

Magazine  (1952)  Page(s) 328.  
 
Hybrid Tea and Polyantha Rose varieties are either budded in summer or grafted in winter, the stocks used being Rosa rugosa, R. rugosa “Spek's Improved' and R. multiflora.

Magazine  (1943)  Page(s) 70.  
 
Opmerkelijk was, dat Margo Koster op R. rugosa niet zo sterk verkleurde als op R. rugosa Spek's Improved.

Translation:
Remarkably, Margo Koster on R. rugosa did not discolour as strongly as on R. rugosa Spek's Improved.