Collecting and study of old bond and share certificates is known as scripophily. It gained recognition in 1970-ies in the West. In Poland it started to gain some limited popularity from mid 1980-ies but it is still fairly exotic hobby with actual number of collectors estimated at figure which is well below 100. It is also very demanding hobby as there are no standardized catalogs and no standardized valuations and in order to appreciate old certificates some understanding of financial market operations is also necessary. So one has to define the scope of activity and spent a lot of time on researching the subject which is not an easy task because number of reference sources is limited, typical collector's literature of this subject sucks (with few notable exceptions those are mostly albums with pictures but with no meaningful information) and really useful information is available only in old stock market reports and stock-exchange handbooks.
Collectible value of old stock and bond certificates depends on a number of factors. The most important value drivers are: rarity, physical condition, age, historic importance and autographs. Rarity refers to availability of particular certificate to collectors coupled with existing demand - very rare piece may not be worth much if nobody wants to buy it. Physical condition is another important factor although not as important as it is in case of coins and banknotes. Collectors of old stock certificates do not have "mint condition" fetish so common among coin or banknote collectors - in practice although some people do follow grading scale applied to banknotes (uncirculated, extremely fine, very fine, fine, fair, poor), there is not much difference between the value of UNC and VF certificate. In my own practice I am using simplified grading scale: acceptable and unacceptable. Certificates showing sticky tape marks penetrating the paper, heavily stained, visibly infected with mould or with significant parts missing fall within the second category. Age: in principle very early certificates command higher prices, while very few collectors are interested in those issued after 1945. Historic importance: certificates clearly linked with famous enterprises, persons or historical events tend to be valued higher than others (examples: certificates of Titanic owners, certificates linked with Panama Canal etc.) Autographs: if original signature of famous person is present then such certificate may command significant premium. Obviously in those rare cases where the certificate is actually valid financial instrument this would also affect its collectible value, however for collector it is not major consideration unless financial value significantly exceeds collectors value. But such cases are so rare that 99,9% of collectors would never encounter such dillema 🙂
Rarity is a major factor affecting the value of collectible certificate however the link between both factors is not as straightforward as it may seem. Eventually the value is determined by play between market forces of supply and demand so relationship between rarity expressed in mathematical terms (as number of available certificates) and value is neither linear nor proportional. This is especially true in the field of scripophily because the number of collectors is quite small (when compared to number of people collecting coins, banknotes or stamps), the highest number of collectors is present in US and it is estimated at ~10 thousand, with Germany coming second with probably ~1 to 2 thousand while in other contries the number of collectors usually does not exceed 100 and my estimate for Poland is ~50-60. This means that market is highly concentrated and focused on certain segments. This also means that even very rare certificate of let's say Bulgarian origin would hardly find any buyers outside Bulgaria and if there are 10-20 collectors there and there are 25 certificates of certain type then such certificate would be considered common despite the low number of pieces actually available. This will be reflected in the price/value of course. From the other hand a certificate available in the same number but of interest to US collectors is going to be highly prized. This means that rarity must be judged in the context of existing demand because without such context it is meaningless.
Official position of Polish Government is that internal bonds (issued by state treasury either in złoty or złoty in gold) are not valid due to statutes of limitation which in case of longest "living" bond expired in 2007. In case of external bonds floated between 1920 and 1937 in gold dollars and pounds sterling all offers of redemption made to holders at various points in time alredy expired. Last one (made to holders of dollar bonds) expired on 31 July 1980. However all those bonds have some value as collectibles depending on their rarity and condition. Their prices are affected from time to time by rumours about future plans to compensate holders or court cases initiated in order to get such compensation however in my opinion the chances that any compensation is forthcoming are negligible.