Russian to English Art/Literary Art, Arts & Crafts, Painting Dimensions

In a paragraph about a painting: the dimensions of the picture are given, and then slightly different dimensions are given in brackets, accompanied by the phrase ' в эталонном списке'. References for this on the Internet generally refer to matters in the socio-political field, and seem to mean a 'typing' of certain categories.

In my context it looks more like a list of standard sizes (i.e. dimensions are given to the nearest standard size), but as the actual dimensions are 50 x 50, and dimensions 'в эталонном списке' are 53 x 50, this seems odd.

Can anyone throw any light on this?
Thank you in advance for any ideas.