Without a close inspection, I can give you some broad numbers for comparison. First, what is the condition of the bores? Second, does the rifle have ejectors? Third, what is the barrel length? Fourth, what is the condition of the metal and wood? I see decent Manton 470's for 19,000 for non-ejector guns. Almost any old, decent double in 470, 475, 450 will be in the 17,000 range. This will be a working gun and not a Rigby, Westley or such. The 465 was Hollands proprietary cartridge and not seen to often in boxlock guns. Condition is everything but a through examination will be necessary to make an evaluation.