T160 Trident Strange Engine Number

I have a 1975 T160 Trident i have purchased as part of a collection. Im a bit confused with the frame/engine numbers. They are matching and don't look tampered with but the number is below the 00100 starting figure.
The bike hasnt been moved for decades and the blue slip is from 1983. The frame and many other parts are plated nickel/chrome but the bike does not look to be heavily modified and is all of a similar age and quite rusty!
Im new to he forum so please pont me in the right direction if I'm posting in the wrong area. Thanks.