Derby "S" roll top desk

Would like to know aprox. mfg. date, see S/N. The top is locked and I have no key. Need help getting it opened. Local lock smith failed, "too old for me" I think it uses a tang key. Generally how many tangs have to be tripped? Help!!!!!

Derby began production of roll tops in the early 1880's, the earlier roll tops were in dark hardwoods such as walnut, the later examples made at the turn of the 19th century were generally made of oak. this one likely dates Circa 1890. Replacement locks are readily available for roll tops, finding a matching key might not be possible, but cutting the lock is relatively easy. The way I've done for years is take two cedar shims and slide them between the bottom edge of the roll and desk top, there is generally about 1/8 inch of play. Slide a piece heavy paper next to the lock tangs ( usually two of them)and cut through them with a hack saw blade. The paper will keep the blade from scratching the interior of the desk.