I have seen tons of items in collections that are unmarked but obvious as to who manufactured them. Can an argument be made in terms of value, that they are not only as valuable as those marked, but perhaps more so because there are less of them?

I think it's totally up to the buyer.

P&L effects are a perfect example. I'm no expert on P&L, but in my humble collection, I have both hallmarked, and un-hallmarked pieces...yet all were purchased from reputable magic auction houses and advertised as P&L magic props (I assumed they were speaking truthfully). Now, the effects that I did purchase which are hallmarked, were described as such, meaning, that must be seen as a definite advantage. The ones I purchased that were not, did NOT include the absence in the description...so, arguably a convenient omission.

I believe Owen was also inconsistent with their hallmarking, as well as Haenchen, and I'm sure a host of others, including more contemporary craftsmen like Mel Babcock. I have a couple of his early die boxes sans hallmark, yet his later pieces have a clear, prominent stamp.

My favorite is Clarence Miller, who faithfully signed - in graphite - almost every piece he produced, with the item number. Gotta love a man who has pride in his work.

I have Miller pieces that are ENGRAVED with a serial number and signed in pen inside (early Impossible Penetrations, pull out the gaffed blade insert all the way and check), stamped...he is the man!

Steve