You must first determine if the stamp was printed using the flat plate or rotary press method. Note that no offset stamps were issued perf 10. If it is rotary press it does not need to be checked for watermark, it is unwatermarked and is Number 543. However, if it is flat plate, you must determine the watermark type.
|Printing Method||Watermark||Size of Frame||Number|
|Flat Plate||Single-line||18.5 x 22 mm||424|
|Booklet (FP)||Single-line||18.5 x 22 mm||424d|
|Flat Plate||none||18.5 x 22 mm||462|
|Booklet (FP)||none||18.5 x 22 mm||462a|
|Rotary "experimental"||none||at least 22.5 mm high (taller)||543|
None of the above stamps are candidates for forgery, they are all quite common.