I am not so sure that it is a modified Alpha 1 frame . I have an '81 GS250 twinshock with a frame number that starts with 250 . I think that the bike listed on Ebay is an '81 GS490 motor in an '81 GS 250 frame . The USA sites that list Maico I.D numbers don't seem to list all of the GS model frame & engine numbers . All of the '81 490 GS frames that I have seen start with 251 , but they are identical to the GS 250 frames except for the frame number .