Note: for the keys in MBGBI it says for females for saxicolella "ostium shallowly U shaped" whilst sternipennella it says "ostium V shaped". Well this isn't totally correct and it all depends on the degree of flattening of the prep. In actual fact for saxicolella the ostium is recessed within the sub genital plate (8th tergite), whilst for sternipennella it is almost flush with the posterior end.
Comparison between Coleophora sternipennella adults and C. saxicolella
1-6 = C. sternipennella (wingspan approx. 12mm, first 2 rows), 7-12 = C. saxicolella (wingspan approx 15mm, third and fourth rows)