Voigtlander Bessa close up lens/filter

I recently purchased and started using a Voigtlander Bessa 120 folding camera which I believe to be a 1935 model. The lens is a Voigtar f/3.5 in nice condition.

My question is about a close up attachment lens. Voigtlander calls it a focar filter that slips on the front of the lens.

What do the number designations mean? The one I was looking at has a number 8. Is that the strength of the filter?

There is one listed on that famous auction site described as VOIGTLANDER 38MM SLIP ON FOCAR 8 FILTER.

Will this work on my model of Bessa?

Anyone have some experience with these focar filters?

Thanks

Sample shot from the Bessa on Kodak Ektar.

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