"CI CB" followed by the numbers 01 51

français vers anglais Affaires / Finance Entreprise / commerce reference abbreviations

These two abbreviations appear together in the references section at the start of an avis d'impôt document. This is how it is set out in the original:
"Trait. [which I have translated as "processing category"] 10
CI CB 01 51
SA [which I have understood to mean secteur d'assiette des impôts = tax assess. div.] 021 ..."
I assume since these abbreviations CI CB were placed together (and none of the numerous other abbreviations in the document are similarly placed together) that they are linked in some way. But when I search for them together I find nothing to indicate what they might mean. While I have found some results for each of the abbreviations individually:
CI: credit d'impôt, code des impôts, code d'impôt,
CB: carte bancaire, carte bleue, commission bancaire,
I am still unclear as to what this combination of abbreviations means and how I should translate it in English.
I would be very grateful if anyone could help me with this.