I came across a sentence that ends with ひとつだけ in a book and searching on jisho gave me zero results (https://jisho.org/search/ひとつだけ); nor did searching for the individual parsing of what I remember from the audio of the VN (i.e. hitotsu--DakE) give me any helpful results that would make sense in context.
TDLR: What does the sentence end(?) ひとつだけ mean in the above context and does it have a general meaning when used at the end of a sentence as opposed to being used elsewhere in the sentence?
Context: Narrator is describing a unfamiliar room he wakes up in. I think that ひとつだけ in the above context will not work as individual parsings as ひとつ most likely means either the "adverbial noun – just" or the numeric one in the above context that adverbially/numerically describes the 鉄扉 (iron door). The next bit, だけ most likely is a particle ("only") but I don't see how the "only" can affect the 鉄扉 as the ひとつ is already doing that to the "iron door". The only way I can see both affecting 鉄扉 is if ひとつだけ is a qualifier sentence (technically a qualifier phrase) being used in reverse-order.