if i had to argue this in court, i would say that this magazine is not detachable even if it is using the california-compliant catch, and not the one pictured above.
my argument would be that, if the magazine was welded, the gun might require tools to disassemble the gun enough to load the magazine. the upper would need to be separated from the lower so that rounds could be loaded.
popping the the takedown pin isn't much different from using a "tool" to press the mag release button. in fact, there are instances where popping the pin is difficult enough to warrant the use of a tool to aide in the breakdown of the gun.
it would be difficult for the prosecution to argue that the magazine is detachable, arguing that the design allows for a detachment, with the aide of some tool. most guns that are designed for any internal or detachable magazine has been made in a way that would allow for regular maintenance of the magazine components. this is a design used by most manufacturers because it would increase the useful life of the firearm, and it allows for better troubleshooting and proper care.
in our case, the mechanism isn't any different using a cali-compliant mag catch which would require a tool to aide in the removal and reloading of the magazine. the law can't simply distinguish between the two without creating a hard and fast rule that either: a) a certain amount of steps must be taken for the modification to be legal; b) it requires the average user to take a certain amount of time to replace the mag for the modification to be legal; c) it requires a certain amount of difficulty to the average person for the modification to be legal; or d) the modification must be so permanent so as to not allow for future maintenance of the critical, moving components of the firearm.
i think the major line should be drawn where we ask whether a person can detach the magazine without the use of tools. if it can't possibly be done without the use of tools, then it should not be considered a detachable magazine. in other words (because i hate double-negatives), if the magazine can only be detached with the use of tools, then the magazine should be classified as permanently attached.
regarding the comment about the ATF, the ATF is a federal administrative agency. they don't really have much to do with state laws because they really don't have much of a say in what the state chooses to define as legal and illegal. the state may try to rely on a definition used by the ATF in helping to define words within the statute however the opinion of the ATF is nothing more than persuasive; it is not binding precedent.
for reference HRS Section 134-8(c) states:
(c) The manufacture, possession, sale, barter, trade, gift, transfer, or acquisition of detachable ammunition magazines with a capacity in excess of ten rounds which are designed for or capable of use with a pistol is prohibited. This subsection shall not apply to magazines originally designed to accept more than ten rounds of ammunition which have been modified to accept no more than ten rounds and which are not capable of being readily restored to a capacity of more than ten rounds.
HRS Section 134-1:
“Assault pistol” means a semiautomatic pistol which accepts a detachable magazine and which has two or more of the following characteristics:
(1) An ammunition magazine which attaches to the pistol outside of the pistol grip;
(2) A threaded barrel capable of accepting a barrel extender, flash suppressor, forward hand grip, or silencer;
(3) A shroud which is attached to or partially or completely encircles the barrel and which permits the shooter to hold the firearm with the second hand without being burned;
(4) A manufactured weight of fifty ounces or more when the pistol is unloaded;
(5) A centerfire pistol with an overall length of twelve inches or more; or
(6) It is a semiautomatic version of an automatic firearm;
but does not include a firearm with a barrel sixteen or more inches in length, an antique pistol as defined in this section or a curio or relic as those terms are used in 18 United States Code § 921(16) or 27 Code of Federal Regulations 178.11.