I have three of these blue filters, which the Hoya website describes as: -

These are color conversion filters for use with artificial light sources. 80A increases the color temperature from 3200°K to 5500°K for the use with 3200°K lamps. 80B increases the color temperature from 3400°K to 5500°K for the use of photoflood lamps. 80C increases the color temperature from 3800°K to 5500°K for the use of clear flash bulbs.

I have two of 52mm diameter and one of 49mm. One of the 52mm filters appears to be perfect, but the other two have very slight blemishes which can be seen at an acute angle to the light, hence the price differences. One of these two is fixed to a 49mm adaptor ring, see third photo. The best price I have found is a special offer of £20 for the 52mm filter.

52mm perfect filter £5 in box

52mm imperfect on 49mm adaptor £2, no box

49mm imperfect £2, no box.

All three for £7 or a sensible offer.

Buyer collects, or could be posted as a large letter.