Need some help finding info on a couple old movie posters, can anyone help me out?

The first is a Frankenstein movie poster w the date of 1931 on it. The other is a King Kong poster w the date of 1933. All I have found out is that those are the original release dates of the films. Both have the old RKO radio pictures emblem on them in the lower right corner. They both are 28 1/2in x 20 1/2in in size. They were both published by Portal Publications, Sausalito,Calf. 94965 but I haven't been able to find any info on the company. I looked on Ebay and found nothing simular or even close to either of the posters....how can I find more info on them hopefully quickly and accurately?