The iron content is probably what Schott glass refers to as "Ionically colored glass" in this glass characteristic PDF of their KG5 HA glass:
Schott glass also writes in it's PDF about interference (dichroic) filters:
"Filters with hard thin film coatings form another part of our total production program.
SCHOTT employs two processes for producing such hard coatings:
1. Reactive ion plating
2. Ion assisted evaporation
Both processes produce very durable thin film coatings. Ion plating, however, gives rise to especially hard and compact coatings. These filters have a negligibly low degree of temperature dependence with respect to spectral transmittance. They are hence suitable for applications involving greater changes in temperature but where changes in the spectral characteristics are undesirable."
"These filters utilize the interference effect to transmit or reflect certain spectral ranges of the electromagnetic radiation. Hereto numerous thin layers with differing refractive indices are brought up to a substrate. The optical thicknesses of these layers are usually a quarter of a given design wavelength or multiple thereof.
When electromagnetic radiation encounters such a multilayer system, the incident beam is split at every interface between two layers of differing refractive indices into a transmitted and a reflected beam. This process is repeated at every successive interface, resulting in the formation of numerous superimposing secondary beams that give rise to interference, either in a constructive or a destructive manner. A wide variety of spectral characteristics with high transmission or high reflection ratings can be produced by varying the nature, number, thicknesses and order of the layers.
The coatings of the interference filters described in this catalog are manufactured by the process of vapor-deposition under high vacuum.
In the case of so-called “soft” coatings, additional measures are normally taken to protect the filters from damage by e.g. handling or from moisture. This is usually achieved by supplementary cementing with suitable glass. The upper temperature limit for these filters is essentially determined by the nature of the optical cement used.
Within certain areas of the UV spectrum it is not possible to use optical cements due to the inherent absorption involved. In such cases the coated substrates are fitted into appropriate mounts and protected by suitable glasses.
In the case of so-called “hard” coatings, the layers of which normally consist of very stable metal oxides, there is generally no need for additional protection. Depending on the substrate selected, interference filters with hard coatings can be operated at temperatures up to about 350°C."