When you say 1851 Navy 2nd, I am assuming that the 2nd means '2nd generation.' I'll base my answer on that assumption.
Steg is correct that there is probably very little difference in value between the '51 'blank' or '51 'engraved' cylinder, though they only made 300 of the 'blank' versus about 3700 of the 'engraved.' Yes, they are somewhat scarce, but their premium over the regular production model is really nothing to get excited about. Within the last month, or two, there were several ads on Gunbroker.com were they were asking $1700-$1800 for a pair of '51 'blank' cylinder with consecutive serial numbers. I don't know what they sold for, or if at all. But, on the average, in NIB condition, a single '51 'blank' cylinder revolver will probably go for around the $700-$750 range.
Recently, I sent you a listing of all the 2nd generation models with year manufacture, quantity made, serial numbers, pricing, etc. Go down the list and look for Model Number F-1100 ('51 'engraved' cylinder - production model) and F-1101 ('51 'blank' cylinder).