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the first machine appears to have a 23" nominal frame size
the second one is likely 21" nominal frame size
fittings such as five-piece steel solid axle hubs, steel bar, steel rims & unserviceable pedals are characteristics of a base model product, much as one might find in a department store offering
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the two Formosan firms most likely to have produced the bicycles are Giant and Merida
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if your parents are curious regarding a specific date for them there are several spots where fittings should carry a date code
first place i would look is back side of crank arms; there should be a date code or even an open date marked there
in the photos they appear they may be a Sakae Ringyo (SR brand) item
other spots to check are the portion of the handlebar stem down inside the steerer, the end of the handlebar, the components of the gear ensemble - the first machine wears an ensemble from Maeda and the second one one from Shimano
there is a guide here which permits one to easily read/decode any markings found; it is organized by the manufacturer brand name of the fittings
The manufacturer's date on a bicycle's components can often be used to determine the date of manufacture of the bike itself. Many bike parts have a date code cast or stamped into the piece. Bicycle component dating
www.vintage-trek.com
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