A lampholder having an inner part and an outer part is formed relative to an axis with a radially extending and axially open slot in turn having an inner portion in the inner part and an outer portion in the outer part. The portions are alignable with each other and both have a predetermined large width. An adapter has a body shaped to fit with one of the lampholder parts and having a formation engaged in the respective slot portion to define therein an adapter slot aligned with the respective slot portion and of a small width substantially less than the predetermined large width so that a lamp pin of a width greater than the small width but smaller than the large width cannot slide through the one slot portion. The adapter body is fixed to the one part of the lampholder.