1851-57 Imperfs - Buying Tips?

Hello All - I am interested in adding a couple of the higher denominations of this imperf series to my US collection. I saw a few at a recent show/bourse but am uncomfortable purchasing anything without a certificate. Are there any basic IDs that a person could use to make sure they are getting, say a #17 and not a #36 with removed perfs? I saw a post recently about the certification of #315 singles is usually based on a minimum heights/widths. Not sure if something similar might apply to the 1851-7 series.

The couple dealers I spoke with said they would grant an extension but if there's something obvious I could check on the spot, it would save everyone's time and effort. Thanks in advance.

My tip for a 17 would be to ensure adequate margins. This SCF topic gives a bit of illustrated info:

https://www.stampcommunity.org/topi...OPIC_ID=5425

Understanding how issues were manufactured to include spacing, perforations etc. is key.

Sc #17 and Sc #36 (as opposed to #36B) were both printed from the same plate, and so are identical except for the perforations. So the only way to know for sure is to have a #17 that has enough margin to show a portion of an adjacent stamp.

"So the only way to know for sure is to have a #17 that has enough margin to show a portion of an adjacent stamp".

Even that wouldn't prove it unless the opposite margin is wide enough to make it certain; it could just be an off center example. But very few trimmed examples seem to be around, since the 36 has about an equal cat value.

I was looking at some #12's and #17's in the Stamp Smarter PSE Cert database and there are quite a few that don't show any portion of an adjoining stamp. There must be a minimum size that they use in order to make the call.

The margins between the positions on those plates are so small, it is all but impossible to find four margin perforated examples.

http://pfsearch.org/pfsearch/pf_grd...lledfrom=lkp

It's really obvious on 17.

http://pfsearch.org/pfsearch/pf_grd...lledfrom=lkp